I'm on letrozole so similar. I haven't gained any additional weight on letrozole (but chemo was a different kettle of fish)
I think most issues are for women who suddenly go into menopause as your weight distribution changes.
I have found losing weight a bit harder though.
My diet wasn't as tidy though as I was more fatigued at night so not prepping good food choices to take to work.
Once I improved that and started recording my food choices in a food diary...well it starting coming off again.
Exercise is helpful...but weight is 80% about what we eat.
Hope this helps. Kath x.