First off, this is a great quality jumper, and the material and colour is really nice.
However the sizing is way off, and it definitely should be advertised as a slim/muscle/athletic fit. I’m 5’10”, I’m definitely not slim, but not really big either, and I initially bought a size Medium, which is what I normally buy in other brands. The size Medium jumper fit like a really tight fitting size Small. I liked the jumper so I returned it and then bought a Large, which was a bit better but it still fit like a tight fitting Medium. Lastly, I returned the Large and then got an Extra Large, and it was finally a decent, and what I’d say is ‘normal’, fit for me.
So, bottom line is, unless you’re really slim or have an athletic build (or just really like a tight fitting jumper), go for at least 2 sizes up from whatever size you would choose based on the sizing chart.