Most of the time it is more about the motor being able to keep up with others rather than what the bike brings to the table. Now if you take a "heavy" lower end womens step thru frame and want to lower the rolling resistance, then by far, tire size should be the biggest consideration. There is an old saying that "saving a pound in your wheels is like saving 5 pounds in the frame": This morning I was out on my steel women's Bianchi with 700 x 35 tires, and had no problem "keeping up" with a friend on his 700 x 25 cc Dura Ace bike.