I personally wear pants over my belly in the colder months because my lower belly can get cold when I wear my pants under my belly. However, when it's warmer I like to wear them under my belly, both because I think it looks a bit nicer and because it helps soaking up some moisture under the belly hang hahah
I have always worn over the belly but I have always lived the wilderness. Winds coming across the Dessert Plateau can mess with the kidneys. Snow can be on the mountains all year around.
This might annoy some of you.
In the harsh conditions it is necessary to eat up to 4,000 calories in a day to stay skinny. That is big eating just to survive.