Sheer size. The penalty there must be minimal. There is not a big difference between both. You'll probably get a few % LF less.
I think there's a feature request, somewhere, to show the amount of penalty. Because the penalty is not the same for a MD90 on a slightly too big road, and my A148 flying from Nantes to Le Lamentin via a tech stop in Horta in 2020(LF Went up from 20% to 90% when I switched to B737MAX & ditched the tech stop).