u can get bigger without lifting heavier weights.
load is only one mechanism by which a muscle can grow.
i went to college with some sprinters on the track team who had HUGE quads and hams and they rarely squated over 225.
the only thing that is going to caus eyour muscle to grow is increased load, whether total weight or repitition. for upper body repitition should increase untill it exceeds 8-10 reps, at which point total weight should be increase so repition drops back down to 4-6 reps. and then the cycle continues until your strong enough to rep that new weight 8-10 times, and you increase the weight again.
lower body muscles tend to respond well to weights/contractions done with higher reps. sprinting, running, jogging, all seem to do very well for developing the legs, at least as far as muscle definiton, hardness, and to a degree size as well. for your srpinters who got huge quads and calves, you dont think those guys's legs experienced progressive overload all the way up to that point? unless that sprinter has gotten faster, or ran longer, then they havent gotten their legs any bigger.