Loss of power

I recently noticed that my brothers Protege felt like it had more power then mine, but we got the same engine, transmission and year. When I first bought my car, it felt maybe a little faster then now but I think it's because I got used to it. I was wondering if weight could change that much the performance of your car... (Mine is fully loaded and my brothers has got nothing) Could someone just point out a couple of things tha it could be.
Thanks for helping!