Could also be an issue of gearing. On a typical stretch of road check to see what gears you are riding in for each bike. I have one road bike with compact gearing and one with mid-compact. They both weigh 17 lb 11oz. Since the mid-compact is new to me I've had to relearn proper gear selection. Both have an 11 cog so when I'm in 50x11 it's quite different than 52x11 in terms of effort.