rotating bones causes model to shrink?

i rigged part of my models legs, but when i rotate them, part of the model shrinks and another bone its connected to moves out of place

does anyone know what causes this?

You could have multiple armature modifiers, could have a bad constraint on a bone, or any number of other causes. You haven’t given us any information about your rig or mesh though, so all anyone can do is guess. Post your .blend file if you want a useful answer.