May sound odd... but I'm kind of the opposite of most, legs for me are 'over' proportioned for me... I would agree with the biking and the running in sand, but if you want other options my major 'bulk' came from jump training (basically its a group of slow and explosive muscle exercises) my best results are from a combination of heel raises and isolated/joined jumps
Heel (or hell) raises - standing still and smoothly raising you body weigh by arching you foot and raising your heels together, its kind of like a SLOW rolling motion forward, emphasis on slow (its not about number its actually about balance and strength). Once at the top slowly drop yourself back down but NEVER let your ankle touch the ground, control that decent just as much as the accent (the muscles remain flexed basically the whole time you are doing this, the moment you touch down, you basically have relaxed your muscles which is not what we are looking for in this portion, this is an endurance push). You can add weight too it but I would start without and move into a light weight, this is almost totally a calf focused exercise... and while it sounds easy try it, the slower you go the more of a burn you start to get as your increase the number.
Combine this with one leg jumps (to isolate a leg) and double leg jumps - simply stand near a wall (you may want some space around you just in case - personal experiences suggest its just good planning...). Orient yourself so you have one shoulder about 1-2 hand width away from the wall and you are oriented perpendicular (and you though you would never need math terms in daily life), take the leg furthest from the wall and put it just slightly behind the foot closest to the wall (this one is going to be 'idle' and is only for balance as you take off and leave). Now that you are basically in a slight step you are going to drop yourself into a variation of a squat, you active foot should be flat to the floor (do not be on your toes!) and leg should be well flexed supporting you weight with your tight as close to parallel with the floor as possible, your knee will be just above you toes or slightly forward. Your other leg will be imitating this stance slightly, but with this leg you are going to be on your toe - again this leg is idle, its basically along for the ride so the more work you do with it the less effective this is going to be.
Your centre of gravity should be over you active foot. If you need drop you hand on the idle side and put it just in front of the active foot for balance if you need a sec or just need to adjust your stance. From here you are then going to rocket yourself upwards trying to convert as much of you force you are pushing down with into an upward thrust, the idle leg just extends almost in a limp state only acting to keep you moving straight up if you have a bad start (keep it in contact as long as the active foot) . As you push off hard from the flat foot, you are going to move you foot from flat into a heel raise smoothly working through the motion above basically (this is the 'twitch' version). Don't roll onto your toes, thats just asking for trouble; you want to leave the ground the front ball of you foot (your toes will leave last anyways but they should not be supporting any of your weight at this point, they are relaxed and getting ready for your landing). While you proceed upwards try to straighten your body, getting all the height you can. Then there is no where to go but down so try to catch yourself with the same foot you pushed off with, try to make the landing as smooth as possible sinking back into the stance you left (as close to a reversal of you up motion - this is where the whole space can be kind of important).
If you have a wall tape you can track how heigh you are getting, but while height is nice, the real goal is to make the form correct with the bulk of your weight being forced up over the active leg. The harder you can 'push' or 'twitch' that leg and move quickly through the motion the better.
(Same for opposite leg, and if you are going to do double legs just use both legs instead of just the one... kind of duh... but better safe than sorry)
Again the exercise is a paring of slow and fast variations of similar motions - the point is to force the muscle through explosive and cumulative endurance to help build strength and size. You can stack them, but i would take a slight breather between the two, especially if you start to seize up slightly... this is often a bad foreshadowing of things that could come....
Big warning other than just the form thing (obvious injury) if you work you calf hard through this, don't do it to the point of a calf lock (its not pleasant). If you get one, try to stand on the opposite foot and then slowly flatten your foot of the effected leg to the floor - don't push it too fast cause thats just asking for trouble, but don't just do the ballet pointed toe because you won't unlock fast and thats where the real pain comes from. The longer you stay locked the more your going to pay after despite how much you think it hurts now; that pain sticks around in a numbing way for quite some time). Also remember to stretch out (both front and back of the leg) after you do this; just because you didn't lock in you work out doesn't mean you won't 15mins, an hour or more after you are done, stretching really reduces this especially if you worked your legs more than normal (there is a reason I call them hell raisers).
If you want harder variations I have some... but tried and true, these work, and they have the lowest 'technique' and chance of injury