How much smaller were medieval farm animals in England than today?

I think both sources copied Early European History by Hutton Webster, published about a century ago. The underlying claim is true: Medieval animals were much smaller than today's. However, it is obvious that "a calf" is not a meaningful unit of comparison.

The historical weight of livestock is mainly determined from archaeological studies as well as records of butchery transactions, and reveal significantly smaller farm animals than today's. See for instance the following figures:

[A]round the year 1000, an adult pig weighed around 70-80 kg, a sheep 20 to 30 kg, and a cow or ox 200 to 250 kg . . . In comparison, at the beginning of the twentieth century, an ox weighed in the region of 650 kg, a sheep from 50-150 kg, and a pig from 100-200 kg.

_{Comet, Georges. "Technology and agricultural expansion in the middle ages: the example of France north of the Loire." Astill, Grenville G., and John Langdon, eds. Medieval farming and technology: The impact of agricultural change in Northwest Europe. Brill, 1997.}

These are from Charavines in France, but English animals would have been similar in size. Based on remains, cattle at York were estimated to be between 220 - 270 kg, for instance.¹

Of course, the weight of animals did not stay constant throughout the whole of the Middle Ages. They were even smaller during the earliest centuries,² and seemed to have gradually became larger close to the Early Modern period.³

In any case, height differences are much less dramatic than weight. Medieval cattle were half the weight of industrial revolution ones, but only 20% shorter.⁴ Hence, compared to ~150cm for cattle and ~75cm for sheep, depending on the species, today:

At Hamwih . . . cattle apparently had a mean shoulder height of 115cm. The sheep were small with a shoulder height of 62 cm.

_{Steane, John. The Archaeology of Medieval England and Wales. Vol. 47. Routledge, 2014.}

That said, only two ounces for the fleece of the sheep is quite an understatement.

The average weight of sheep fleeces per animal on the Winchester manors from 1300 to 1324 was 1.5 lb.

_{Clark, Gregory. "Labour productivity in English agriculture, 1300-1860." Campbell, Bruce MS, and Mark Overton, eds. Land, Labour, and Livestock: Historical Studies in European Agricultural Productivity. Manchester University Press, 1991.}

Seasonal variations aside, differences in fleece weight were mainly region dependent. A particularly poor area was East Anglia, and especially from the pastures of Breckland.⁵ Yet, even in Breckland the worst yield was still about ~1 lb, or 16 ounces:

^{Bailey, Mark. A Marginal Economy?: East Anglian Breckland in the Later Middle Ages. Cambridge University Press, 1989.}

Notes & Refernces:

<sup>1. O'Connor, Terence Patrick. Bones from Anglo-Scandinavian Levels at 16-22 Coppergate. Council for British Archaeology, London 1989. "[For] a very lean conformation, an average liveweight in the region of 220kg would seem likely. For a heavier conformation, this average could perhaps be raised to around 270kg."
2. Crabtree, Pam J. "West Stow, Suffolk: Early Anglo-Saxon Animal Husbandry". East Anglian Archaeology Report 47. Suffolk County Council, 1989.. "Based on the measurements of the trochlear breadth of the humerus, it is estimated that the average West Stow cattle would have had an average live weight of only about 150-170 kg, and a fat-free carcass weight of about 100 kg.
3. Kershaw, Ian. Bolton Priory: the Economy of a Northern Monastery, 1286-1325. Oxford University Press, 1973. "[T]he average carcass weight [was] about 430 lb. for oxen urhcased for victualling the navy in 1547."
4. Clark, Gregory. "Labour productivity in English agriculture, 1300-1860." Campbell, Bruce MS, and Mark Overton, eds. Land, Labour, and Livestock: Historical Studies in European Agricultural Productivity. Manchester University Press, 1991."[N]ote that cattle in this period were about 80 per cent the hiegh tof cattle in the late eighteenth century, which would impaly that they were about 49 per cent of thre weight."
5. Bailey, Mark. A Marginal Economy?: East Anglian Breckland in the Later Middle Ages. Cambridge University Press, 1989. "Breckland's poor pastures were not conducive to producing heavy, thick fleeces, and its sheep were of the shortwool variety whose fleeces were lightweight and low in quality. This was true of East Anglia in general, but it would appear that Breckland fleeces were poor even by these standards."</sup>