羊? + 大 = 美
How to understand the origin of 美：
A big fat sheep is certainly nice to eat.
It’s a beautiful thing to eat the sheep meat.
Sheep are the main animal to provide meat for people among the 六畜 six domestic animals. The six domestic animals in ancient times were horses, cattle, sheep, chickens, dogs, and pigs.
So, the ancient people considered sheep beautiful and then created 美.
That’s why there are expressions such as 美食，美味，美餐，etc.
The original meaning of 美 is similar to that of 善 and 義.
文言版《説文解字》：美，甘也。从羊，从大。羊在六畜主給膳也。美與善同意。
白话版《说文解字》：美，甘，爽口。字形采用“羊、大”会义。羊在六畜之中是提供肉食的主力。“美”与“善”同义。