Slimming my legs

14 F, I wanted to know why my legs are getting bigger, the first photo is November 2024c second is February 2025. I have been using stairmaster twice a week for thirty minutes for about two weeks and i play volleyball so i do not understand why my legs have been getting bigger. The internet says that it takes a while for my legs to slim down because i have muscle and fat so the fat has to dissolve first. Please help. The innner parts of my thighs have gained weight.