Working a muscle never makes that area "thinner." Biking builds your quads and hamstrings, therefore making them more dense and even larger.
However, cycling has the greatest benefit in that it works those large muscles, therefore increasing your metabolism (making it easier for your body to burn more calories ALL DAY).
Biking (or swimming or running or hiking) will also make you get a flatter tummy and lose fat because you're increasing your metabolism.
It's a common myth that "doing situps will give me flat abs." That is bogus. Doing situps works your abs but isn't any more beneficial toward losing fat than cycling, running, etc.
Cycling will possibly change the shape of your legs -- tighter calves, more defined hamstrings, and a bit of "lift" to your butt. But this varies for everybody!
