Need a little more information than that.
Did you change any game settings between changing LCD's?
But in reality it might not be as much of a "weird" situation as it might be. The CPU is used much more in lower resolutions (and 1440x900 is pretty low for a desktop), and in higher resolutions, the GPU get's pushed more than the CPU (well it depends on the resolution as well).
Maybe it's just that you have been wowed by a larger, nicer screen