Spanish and SOME EU sizes, at least for men, run about two sizes smaller than usual. For example, I wear an XL t-shirt or polo here in the US. But when I show up at the Pilgrim Office for my annual monthly stint, my t-shirts are XXXL. This gives me the equivalent fit to the XL. I do not like fitted outer garments.
I am only about 179 centimeters (I shrank from 185). But, I share your girth, at 113 kg. Yikes. It is relative, but my torso shape is that of a beer keg...see avatar...
That said, I have bought men's Adidas branded polos in XL at the Santiago Decathlon that fit perfectly. If I buy the house brand Quechua, I usually have to go up one size, to XXL, just to get it to fit. If I want a roomy fit, my preference, I must usually go up two full sizes.
So, if you are trying to buy the least expensive Decathlon-brand shirts, start at the high end, try it on and hope for the best.
Note, and for the record, I have had the same problem when trying to buy nice sweaters in Italy, and in France.
Hope this helps.