Mine is from 2021 - maybe that's the previous generation? Anyway, it's fairly loose on the arms and midriff and wearing a fleece (Nitro) underneath is no problem. It's tightest at the armpits. A down jacket (
Incredilite Endurance) fits under but fills the Helium at the upper chest and armpits. For reference, my waist is narrow and chest relatively wide, slim overall.