为什么使用更大的位图会在我的游戏中产生更多内存？

My game's initial heap was 12 Mb.now after loading a bitmap of size 1280*800 heap size has increased to 22 Mb.My game isn't even complete yet.just some basic setups.I am afraid memory will reach to a great extent this way. Also I was playing some game yesterday,it was also using so many diff images of same size but its size was only 20 Mb.how is this possible?My game's initial heap was 12 Mb.now after load