I was wondering if you can post a picture of your legs? I mean do they look like you are riding a chicken? Do you just want huge tree trunks for thighs?
No ones legs are bigger than serious weight lifters, but can you imagine them trying to ride a bike?
Bike sprinters and ice skate sprinters, running sprinters all have big legs. Usually, the runners' upper bodies are also larger, specifically their shoulders, pecs are larger because there is a corolation to how fast they move their arms in relationship with their legs.
Big gears and hills will get your thighs bigger, but if you are riding too long, and just plain overtraining, and undereating (right foods of course), you will begin to burn your hard earned muscle.
Just my dimes worth.