Does riding a bike with smaller wheels burn more or fewer calories than riding a bike with bigger wheels?

I track my exercise, and while I know that calories-burned counts are always an estimate, I'm wondering if I should mentally round up or down.

Let's say I ride for 45 minutes at 10 mph. Do I work harder or less hard on a bike with 20" wheels than I would riding for the same time at the same speed on a bike with 26" wheels?