do some leg extensions where you sit with weights on your legs and swing them, its normally in most gyms its a machine
Generally machines suck, but if you are simply trying to add mass as opposed to focusing primary on functional strength, don't be afraid to abuse the leg press machine, in addition to free weight work.
Squats and deadlifts, and if you want size you could try 3 sets of 10 on the heaviest weight you can do 3 sets of 10 with. There are probably plenty of supplements and protein shakes that would benefit you as well.
Speed Skaters have HUGE legs. Perhaps you could take up speed skating. Or maybe, it's just that the people with small legs just don't go into speed skating? Jump squats are good exercise.