motta,
Believe it or not, the push-up, just like the bench press, is more of a tricep-building exercise than it is a chest-building exercise (this, of course, depends on hand positioning, but it is still primarily a tricep exercise). That is one of the reasons you are not getting as much growth in your chest as expected.
Secondly, high reps will not result in high muscular growth. If you don't have access to a gym, head out to Walmart and buy yourself a backpack. Stuff some weight in there. Whatever you can find - rocks, dumbbells, barbell weights - whatever. Put it on your back and start doing lower rep pushups that way. Keep it between 8-12, and increase the weight once it gets too easy again.
Note: You will not look even remotely close to He-Man by simply doing pushups. You'll need a gym for that.
-Warik