My strength is going up but I am not getting any bigger? 10 POINTS !!!!!?

I am 20 years old 5'11, 145lb I can bench 270lb, dumbbell shoulder press 80lb dumbbells, tricep extenstion 120lb dumbbell (2 hands), bicep curl 55lb dumbbells and leg press 520lb. I feel like I always push myself to the max and train heavy and intense. The thing that I do not understand is that my body frame stays the same I get stronger but I do not get any bigger as you can see from my weight 145lb. Any advice would be appreciated.