Yup. Some people use more foam than others. Most people use gray, but I've used white as well. ANH (AP, TE2, etc.) I've found requires more foam to get it to hang/look right. I have some in my back plate, butt plate (I have no an impolite person), and biceps. On my FX I just use a little in my forearms.
Some really skinny people even wear a muscle suit underneath.