Just take it one step at a time. Everyone's bodies are works in progress. I worked up to 230 lbs and stayed there for a few years and just recently have dieted back down to 200 and everyone is asking me how I put on so much muscle not realizing I was carrying/building the muscle all along but losing some fat made it more visible.
You may be surprised that you don't lose any roundness but gain it. Your butt may lose a little size, but the shape would likely improve.
If you're set on getting it bigger and rounder I'd suggest hitting your glutes from all angles: squats, deadlifts, stiff legged deadlifts, hip thrusts, cable pull throughs, lunges, drag sled etc. You need to make sure you put your glutes under tension while extended (deadlifts & squats) and also in flexion (hip thrusts, cable pull throughs).
Nice work so far though. You look better than 90% of women right now.