generally speaking,large legs give more short term power,and slimmer legs give longer endurance.Like sprinters tend to be more muscular and long distant runners slim.But the anomaly is to get large legs heavy wieght and low reps give best results,yet speed can be related to power,so its not as simple as it seems,it all depends what you want to train for,these can be reversed for example if you bodybuild for large strong legs,ie low heavy reps,but go running too.The upshot of it is,nowadays you can really design your muscular shape in most sports by training and eating correctly in whatever way you like,if your legs are getting too big ,lower the wieght and up the reps,or less protien and lighter running.Some people are naturally large in musculature others are not,like i said its just in general.