I have the same problem.

When I started riding several years ago, my son was smaller than me and he couldn't keep up with me going uphill or downhill. Then one spring, we went on a family bike ride and he was taller than me. I'd been running and exercising all winter and he'd been much less active, but except for going uphill I couldn't keep up with him.

Also, my husband is ten inches taller than me and on several occasions he's been behind me going uphill and he coasts past me going downhill (says he just stopped pedaling at the top of the hill) while I'm still pedaling my butt off.

Size is definitely a factor. (I kind of enjoy going uphill for that reason!)

Deb