How did Maitreya become so large and rich?

The sarcasm seems unnecessary, monks are often not allowed to handle money, even if a lot of sutras describe opulent buddhas.

All the statues I've seen first hand of Buddhas are just relics, and of ascetics: so maybe you could look into the history of Buddhology etc., as well as looking into different cultures.

I think the image on the Chinese Wikipedia page is captioned "laughing Buddha" which (e.g. according to the English Wikipedia) is a nickname for Budai/Hotei:

He is almost always shown smiling or laughing, hence his nickname in Chinese, the "Laughing Buddha" (Chinese: 笑佛; pinyin: Xiào Fó).

I think I read somewhere/sometime that people would give him pennies (small coins) which he'd then give to children.

I guess he's shown surrounded by treasure because he gives it.

Anyway I think this is a duplicate of Why is Budai often associated with the future Buddha Maitreya?

I think that the Chinese Wikipedia page you referenced even explains the connection with Matreya, see also 布袋和尚 -- i.e. that the monk left a verse which said something like ...

"Matreya has thousands of billions of incarnations, always shows himself to people, sometimes people don't know [him when they see him]"

... which gave rise to the belief that this monk was an incarnation of Matreya.

The English Wikipedia article (referenced above) also says something about how he was imported into the Japanese Zen pantheon in the 12th century:

The Chan Masters, Dahui Zonggao (1089–1163) and Hongzhi Zhengjue (dates[clarification needed]), were both leaders in the initial merging of local legend and Buddhist tradition.[3] They hoped the induction of likeable and odd figures would attract all types of people to the Chan tradition, no matter their gender, social background, or complete understanding of the dharma and patriarchal lineage.[3]

The Chinese Wikipedia page says something like,

The famous couplet:

"The big belly can tolerate the world's difficult things, and the mouth will smile and laugh at the ridiculous people in the world"

then the Bodhisattva's broad mind and optimism are portrayed vividly and vividly.

So I guess the big belly is meant to portray robustness, not subject to misfortune, or something like that.

In Buddhism, the Buddhist statues are essentially the artful and symbolic expressions of the Buddha teachings, with the objective to educate us the meanings and teachings of Buddhism. Therefore, we should see the statues of Buddhism as teaching aids and the cultivation of virtues we aspired to achieve. They are not meant to be statues of gods for worshiping. As we come to realize the inherent meaning of Buddhist symbolism, we will truly appreciate the sophistication and completeness of its educational values.

Maitreya Buddha, or the Laughing Buddha, or the Bu Dai monk (the historical monk figure better known in China), has widely been seen to be portrayed with immense happiness, and its statue is sculpted big in front entrance of a Buddhist temple, and is also sculpted as fat and happy with a big smile and a big belly.

The educational and metaphorical values of the happy and fat imagery of this sculpture is created with profound significance:

1. The Laughing Buddha is often found at front entrance of a Buddhist temple. This shows that Buddhism greets visitors with a hearty smile, welcoming anyone to come on in.

2. Buddhism teaches us how to get rid of sorrows and afflictions and attain happiness in our lives. The laughing statue shows visitors coming in to learn Buddhism and follow the Buddha path will attain a happy life. We should also learn to be cheerful and be courteous to all.

3. Buddhism teaches us to be tolerant and treating everyone fairly and equally. The big belly represents the attitude of inclusiveness, that we take in and be tolerant of anything good and bad we experience and anyone we encounter in our lives.

4. Some statues portrayed with riches are the artistic representations of our true inner nature of perfect, complete, and infinite treasures (metaphysically) that we all beings by Buddha-nature possess.

We can see that the above profound significance of the sculpture are actually in sync with what Buddha Sakyamuni has been teaching all of us some 2,500 years ago when he reached enlightened in this world and educated us what he is the path to enlightenment.