why do all models have small ....?

heads?

for runway/fashion modelling small breasts ae preferred as the model looks more elegant than if she had a huge rack while walking down the catwalk...

there are many diff types of modelling though, some which prefer larger chests.

Stomachs? Two words: Anorexia Nervosa.

Waistlines?

Indeed Slim, every single models do have small dots dots dots dots. You have cracked the case.

Waist lines

Kidneys?

It's part of the "high fashion" look. That's why they are usually super tall and freakishly thin.

Because they are slim &/or thin. Not all models are anorexic.

Sometimes big boobs can mean high bodyfat.

Because they can't have them to big. My opinion though I prefer Double D's.

Bikinis? Well, they're umm.. what's the right word? well, you fill in the blank. ______ have a nice day! :)