I recently bought a pair of ProChaps Half Chaps, and they are AWESOME. I have very big calves and the XXL's I got were tight to get on, in fact it took DH and I to squeeze me into them the first couple of times

Now they go on and zip up nicely, so I would vote for the smaller pair, they should be more comfortable as they stretch and break down a little behind the knee.
If they are leather then I would treat them the same as a new pair of long boots, get them on, sit in a bath of water until they are soaked through then walk around all day in them, they will conform to your legs and will feel a lot better behind the knee.