I had the Vega monoflap. The narrow tree was VERY narrow and the medium was true to size, if anything a bit wide. It didn't fit any TB I put it on and I was riding 8 horses a day in BNT's barn of 60+ horses. It's more suited to hefty WB types.
I have an Amerigo (not Vega - older) CC monoflap that is marked a M and fit a TINY bit narrow before being adjusted. Because these saddles can be adjusted to an extent, it's possible that if you have been trying used, they had been let in or out in the past. Don't have much experience with newer models, though.