Just wanted to let my helpers know (the ones that responded with useful ideas about why my stirrup leathers were hurting my thighs, since I'm just now back in the English saddle after several years).
It was too small in the seat. For one, the person I bought it wrong must have mis-measured, but after the useful tips I got here, it was waay too small for my post-baby bottom. I have another one, and I don't have that problem any more. My stirrup length was fine, too. I was just scrunched up in the tiny seat.