pleez help n tell sum principals of buddha

pleez help n tell sum principals of buddha

I’ve seen some of these Buddha statues that have huge bellies, rub for luck etc. When I look at Buddhist statues in temples though, the Buddha is thin with hair on the head. Are there different ones?

I am taking a history of Asian Art class and noticed that many of the ancient Buddha statues are smiling such as the Colossal Buddha at Yungang. Is there a particular reason for this, not that its a bad thing or anything, just curious.