Can you change the breast and waist sizes on Sims 2?

No, just when you are creating the sim you can chose if they are fat or thin.

or else when you are playing, and they are fat, and you want to make them thin make them exercise (swim or work out) and if you want to make them fat, make them eat a lot and dont do exercise...

in sims 3 you might be able to do what you want to do... I dunno...