Wealth often equals gluttony. Not an attractive trait, I must say.
You sound like an ultra-puritan calvinist. Fat could have its own grace, its own beauty. Mussa is, for me, perfect. The way FRX wanted to depicted Mussa was like a man with amazing wealth and, as they say in the first look, he spent his wealth to improve the life of its people. He was a man of strong faith, but faith for Islam can (and should) mean hospitality, helpfulness and generosity. So, since we have no official, historical depiction of Mussa during his reign, Firaxis has to make a character that match its personality, therefore a man ready to help his people and thus jolly, smily and (because he were filfthy rich) fatty. Islam is not a religion against fun, opposed to what people might say. Mussa could really have looked like this.
Besides, you look quite contradictal: you don't like Gengis Khan because he
doesn't look like what he
should be (a villainous schemer) but you don't like Mussa neither because he
does look like what he
should be (a generous, wealthy, faithful islamic medieval leader).
(And for your remark about the fact that, without muscles, you die in this time, I should remind you: Mongols fight mainly with archers and bows, not with closed warriors and swords ; muscles were not as important as stability and precision. It was not the english longbow, which required huge muscles).
