Hip bones won't widen once the growth plates on bone ends stop working, somewhere between 18 and 25. However, estradiol does alter the growth pattern of connective tissue, including tendons. After we are on HRT for a while, we can expect the changes in connective tissue to rotate the hips forward a bit. This gives us a little joint pain and some soreness as other tissues adjust.
I bet that's what you are seeing.
The same effect is what slightly alters height and shoe size, all other things (like weight) remaining constant.