Why is a pocket-battleship large in size ?

Wikipedia's article on the "pocket battleships" covers this. The ships were designated Panzerschiffe ("armored ships") by the Germans, and the British started calling them "pocket battleships" because of their armament (barely battleship-class) and their unusually small size for the armament. As warships went in the period, they were fairly large, but much smaller than contemporary battleships.

A typical Battleship of the era ranged from 25-45,000 tons, so yes, for a Battleship these were quite small. In fact, that was exactly the point.

The German Pocket Battleships were an artifact of the naval restrictions in the Treaty of Versailles. The Germans were prohibited from building ships displacing more than 10,000 long tons, which was considered the weight-class of a heavy cruiser. This was designed to prevent the Germans from building any more battleships.

So when the Germans started rebuilding their Navy, they wanted Battleships of course, but these were the absolute largest ships they thought they could get away with building. That's where the "pocket battleship" came from.

Now I know what you're thinking...15,000 is larger than 10,000, right? Well, the Germans didn't exactly like the Treaty of Versailles, and weren't enthusiastic about sticking to it to the letter. Where they could fudge things a bit, they did. What are you going to do about it, go to war? Over a slightly too large ship?

Everything is relative. Someone can be poor, compared to Bill Gates. And rich, compared with a peasant from Ethiopia.

A pocket battleship was not really small. They were capital ships in their own right and second only to battleships. They were relatively smaller. So why not build full size battleships then?

1- Money. Them ship are darn expensive! You can build a pocket battleship faster and (perhaps) more of them in the same time to build a full sized battleship. If you have enough slipways and trained staff to build them, that is.

[This is something almost always overlooked when people think Germany could have won WW2 if they only .... fill something in. In order to build more battleships, you need more slipways. Germany had only 3. One of them was used to build the Graf Zeppelin. Apart from having enough slipways you also need materials and skilled engineers to build. You can't use them elsewhere. So more battleships means less something else. Submarines, for example.]

2- As with tanks you have 3 factors deciding how successful a ship (or tank) is: speed, firepower and armor. It's very expensive and difficult to build a battleship that is the fastest, best armored and best armed. The Japanese did that with the Yamato and the Musashi. They could only build 2 of them, and they didn't amount for much during the war. Not a fault of the ships, mind you. They were already obsolete on the drawing board, but nobody realized that.

The Germans opted for fast and well armed, but less well armored. The theory was that almost all ships were less well armed than a pocket battleship and could be defeated. If they met a better armed ship it would be a battleship and they could outrun it.

3- Treaty obligations. After WW 1 nobody wanted another naval arms race. There were too many big ships, and money could be spend better elsewhere. Most nations agreed to it. Except Japan later on, and Germany had to start from scratch. Apart from the fact that they weren't allowed any capital ships at all. So a pocket battleship was a good way to begin building capital ships. A pocket battleships sounds less intimidating than a battleship.

• Which brings me to your question: why is a pocket battleship so large?

That's because of the artillery. It needs to be that large in order to house 11 inch or even bigger guns. I'm not an artillery expert, so take my opinion with a grain (or a ton) of salt here. The bigger the caliber, the proportionally larger a ship needs to be. The difference between 8 or 9 inch gun on cruisers and 11 inch or bigger on pocket battleships is huge.

There have been trials with upgraded cruisers and some upgraded cruisers were commissioned, but they were a failure. You can't beef up a cruiser to do the smaller work of battleships. You need smaller battleships for that: pocket battleships. Even so, the concept from its early beginning was criticized. Jacky Fisher who thought up the idea couldn't really specify what exactly a pocket battleship would be useful for.

You didn't ask for it, but Germany planned rearmament almost immediately after the armistice. What Hitler did was to turn the switch from covert to full power. Even if Hitler hadn't been elected, another government would have done almost the same thing. Perhaps a bit later and less aggressive.

Question:
Why is a pocket-battleship large in size?

Battleships are measured not by length but by displacement and by that metric pocket-battleships aren't large compared to their contemporary battleships. A WWII era Iowa Class battleship weighed 45,000 tons. A British Battleship like the the HMS Hood weighted 47,000 tons. The Japanese Yamato displaced 65,027 tons. WW2 era battle- ships dwarfed the roughly 10,000 ton displacement of the German Pocket Battleships.

The Treaty of Versailles which ended WWI limited the weight for German Battleships to 10,000 tons or about 20% of a larger British Battleships. This was supposed to limit their lethality and give the WWI allies an advantage against Germany in future wars. Germany responded to this limitation with building the Deutschland-class cruiser from 1929–1936 which incorporated a series of innovations which reduced the weight of the ship. These innovations included all diesel propulsion, and wielded rather than riveted hull construction. These German ships made up for their lite weight by carrying heavy armaments, six 11 inch guns which made them formidable despite their reduced tonage.

Deutschland Class Cruiser
Due to their heavy armament of six 28 cm (11 in) guns and lighter weight, the British began referring to the vessels as "pocket battleships".

Sources:

• Deutschland Class Cruiser
• history of welding in Shipbuilding
• Welding: History
• Treaty of Versailles
• HMS Hood
• USS New Jersey
• Yamato

Comments:
@Noeshel: Do you mind explaining it to me what “displacement tons” means ?

If you have a 45-70 ton ship you can't very well put it on a scale to measure it's weight as one would with a sack of flour or basket of fruit. It's impractical right. If you know the weight of cubic foot of water.

       1 gallon weighs 8.3453 pounds.
       1 cubic foot of water contains 7.48052 gallons
       1 cubit foot of water weighs 62.4272 pounds

Based upon how low the ship sits in the water you can calculate the volume of water it displaces. The weight of the water displaced is equal to the weight of the ship. As Solar Mike stated, we know this from Archimedes who famously performed the same experiment in his bathtub in antiquity.

Now you can add to this complexity with short tons and long tons. The United States uses a different standard and their tons are lighter than British Tons. Short ton represents American Tons and Long Ton represents British tons.

Then as @o.m previously stated, their were all kinds of games played to maximize or minimize weight. Weighing the ship fully loaded with fuel and or armaments could make the ship seem larger, or the inverse could make it appear lighter.

At the time, various naval treaties tried to limit arms races. The London Naval Treaty defined light and heavy cruisers as ships with less than 10,000 displacement tons, armed with 6.1" or smaller guns (light cruisers) or 8" or smaller guns (heavy cruisers). Larger ships would have been capital ships (i.e. battleships or battlecruisers). Modern battleships at the time were around 35,000 displacement tons.

The Deutschland-class was around 15,000 displacement tons fully loaded, 11,000 displacement tons standard. That makes her a "pocket" battleship compared to other battleships.

Note that according to the Versailles treaty, Germany was only allowed ships up to 10,000 tons, and they claimed that the Deutschland-class was within this limit. Her 280mm armament and the armor made her a capital ship under treaty definitions, not a cruiser (even if she was cruiser-sized).

(A floating ship displaces water. Measuring the weight/volume of displaced water is basically a measure of the weight of the ship. Displacement tonnage may be listed fully loaded and fueled, or with partial fuel, or unfueled. Many naval powers tried to play games with the way the calculated tonnage for official records.)