Exercise is very important for maintaining muscle mass and bone strength while on hormone therapy, but you need to do an awful lot of exercise to burn up a small amount of excess weight, so dieting is going to be more important for keeping weight in check.
I found purchasing body composition scales to be very useful, so I could monitor my muscle and fat composition. For example, I was pleased to see my weight reduce at one point, until I realised it was due to a loss of muscle and gaining fat. That caused me to increase the protein proportion of my diet.