Yes, when you measure the widest part of your hips it includes the buttocks. At least that is what I learned at school. This picture
kind of shows where you have to measure. If you want to, you could pad the buttocks too, but it's not a must. But if you are wearing something that's very tight, you'd look more feminine if you padded the butt too.
At I con I went to last summer there was this guy who did an amazing crossplay of Bayonetta,
which is an incredibly tight costume. And he had the most beautiful butt xD But he confessed that he had used some kind of padded panties like this.
So that might be an idea.