the thighs and calfes bigger instead of slimmer?

Would I burn the same amount of calories using the eliptical trainer for 30 minutes and running on the treadmill for 30 minutes at 5.8-6.0 mph?

On my boyfriends electric treadmill I can. On my manual at home whihc is narrower and at an odd unadjustable (w/o the use of outside forces such as books under it which I may try) angle I have a hard time doing this. Is it really affecting my worksout?

I love the treadmill but I have been told that it can damage your knees significantly due to the high impact. Is this true? If it is, is there anyway you can lessen the impact while still using it?