Different brands use different size guidelines but the terms remain the same
By letting ambassadors try and review jerseys from many different brands we get an idea about which brands follow which guidelines when it comes to sizing. And it seems like there is a very strong correlation between the country of origin of the brand and the way they go out about using sizing. What can be slightly confusing is that all brands increasingly go by the US sizes S, M, L,XL , XXL etc. But it is not always that obvious what is meant by any given size. adidas holds US and EU sizes to same constant. But has Asian sizes one bigger. So EU M is an Asian L. Macron the Italian brand makes a distinction between US and EU sizes. so a Macron EU M would only be a US size Small. Since the population on average is taller to the north of Europe (and North America) usually sizes tend to be larger from brands with origin in these geographical areas. And on the reverse. People from southern Europe and from Asia tend to be shorter on average thus those brands come slightly smaller.
The Italian brands:
The Spanish brands:
The Chinese brands:
Tend to favor shorter jerseys but possibly slightly broader relatively.
The brands from the north:
tend to be larger in size. There is not always a 100% correlation between country of origin and size guidelines. Puma for instance used to come very small in the sizes but subsequently changed their measures. Joma and Legea can come quite big in some of the men’s sizes. But as a rule of thumb there does seem be significant backing for applying this perspective when evaluating the sizing online.