I had to size up as well. Upper arm was like a sail. Good resistance training for sure. Plus felt like in a sauna. Sold it immediately and bought the shakedry.
I use it for running too. That has good ventilation superb light but very fragile so should never wear a backpack over it
So yeah, I was completely wrong. It’s tight all over other than the upper arm but that’s probably more to do with my pipe cleaner arms.
50cm pit to pit, 38cm waist hem. Very unlikely to fit anyone who normally wears large given I am small in most Rapha.