Didn't have to size up
As reference, this fits similarly to the Patagonia R1 full zip and retro-x vest where I'm a medium (5'10, 165 lbs, 32 waist).
If the vest is layered over the R1, the R1 hangs about 1 inch longer in the back. So the tech vest runs short and will have a very slimming look, but nothing I would complain about.
I initially ordered a L but it was a bit too baggy, particularly around the stomach.
This fits close to the body so I can't put a thick hoody under it, but this layers very well over a t-shirt and light midlayer, and under a ski jacket.
If you have a gut, then I'd order a size up, but otherwise, get your usual size.
Quality is great as expected from MHW.