Not much to elaborate on. Friction force equation is mu*N, coefficient of friction times the normal force. Area plays no role.
Work is force times distance, two engines of equal displacement, the one with a longer stroke will do more friction work, or put another way will have more friction losses.
I would also argue it's tougher to keep a consistent ring seal over a greater distance as well..
All theoretical of course, but these are the kinds of things which can explain what we observe when the tests have been done.
