Got a new bike, had sram rival put on it with a compact crank and an 11-28 cassette. My other bike is a 9 speed compact crank with an 11-25 cassette. I have been riding my other bike on a trainer for a few weeks. I finally got pedals for my new bike so i put them on and placed it in my trainer. It seemed so much easier and smoother in the highest gear compared to my other bike. Haven't ridden my new bike outside yet.

Any ideas why?

Gearing that much different?
Different tires?
Going from a 9 to 10 speed?
Bottom bracket different?
I'm certifiably crazy?