At the battle zone beat your rival 100 times and you will be able to enlarge it. Heres some more stuff:

Beat him 150 times:get a gorgeous gem.
Beat him 200 times:get a pokemon trophy (bronze).
Beat him 250 times:get a pokemon trophy (silver).
Beat him 300 times:get a pokemon trophy (gold).
Beat him 1000 times:unlock hoenn region, you can go there and explore.
Beat him 1500 times:unlock johto region, you can go there and explore.
Beat him 2000 times:unlock kanto region, you can go there and explore.

Yes this seems impossible but its not. Just go every few minutes and challenge him. Oh yeah- if you beat him over 10,000 times your game will get a serious glitch.