Welcome to the forum, Luna! BTW- 25 miles a week is great at the beginning. A year ago I started riding by doing 3 miles, then 4 etc. I now ride about 75 miles a week- and hope to do more.

I slimmed down pretty soon after beginning to cycle. I would definitely not worry about cycling making your thighs bigger - mine are more toned and muscular now, but less fat- in fact I need some new clothes because they are too loose now . So, like others said, check your food intake - if you are doing a lot of "carb loading" and not cycling long miles yet, that could be your answer. Just keep going and having fun out there.