One can often get impressive leg-size with basic leg exercises by either lifting heavy with low-reps (1-10, usu. with multiple sets) or by using a moderate-weight with high-reps (15-20, multiple sets), or a light-weight with very-high reps (usu. around 100 reps, usu. for only 1 set... if seated at a leg-press station, then can go much higher with the reps -- 300-500 -- if the weight is relatively light).
There are also quite a few non-weight (aka body-weight) exercises that can be done in multiple sets with high to very-high reps, such as a body-squat, heels-up deep knee-bend, sissy-squat, 1-legged squat (if/when you're strong & stable), body-lunge (forward & backward), & various squat-hops. One can also add a couple of dumbells in with these exercises for added resistance. Other (often) non-weight exercises include various leg-lifts & a favorite thigh-killer: a 3-point-(pivoting)-kneel-walk.
You can also hit the hams & glutes more directly with various exercises.
There are many exercises & general activities that will cause your thighs to balloon, but some may be hard on the spine, knees, ankles, &/or hips, especially if done in/with excess. Among the many activities are mountain back-packing, dancing, skating, sprinting, & cycling.
And it's best (especially in the long-run) not to neglect your other body-parts.