Have you measured your legs? The eye can be deceived.
When you exercise, make sure you are using the same sets of muscles for both legs. sometimes if you have a weaker leg, you may have a tendency to use the upper front thigh muscle more, because that is often the strongest muscle of the leg. that can cause it to overdevelop. (I used to be in a ballet company in my younger and thinner days!). so if it really is bigger, just be more conscious of how you are actually using and supporting your legs as you work out. if you are favouring one leg because it's weaker, tone down your routine to accomodate your weaker side until the strength is the same on both. after that it will naturally sort itself out after a few weeks and i think you will notice the size is the same too.
in other thoughts, maybe you are retaining water in one leg more than the other.