This shirt, the Proton, is one of the very few Mission clothing items I didn't own previously. When ordering it, I saw Mission suggested moving one size up from normal Mission clothing so I followed that. While the XL does fit me fine, I'm fairly sure I could've gotten away with the large. Either way, I absolutely recommend this shirt, if you don't want to read my full review. However, I'm going to include exact measurements below and directly compare it to the Stratus, Mission's other running specific shirt and the one they don't recommend moving up one size. Measurements: Proton size XL(This review and the one Mission recommends one size up): Pit to pit- 22.5 inches Top of v collar to bottom front- 25.25 inches Top of collar to bottom back- 29.5 inches Stratus(Mission's other running shirt and the one Mission says runs to size): Pit to pit- 21.25 inches Top of v collar to bottom front- 24.75 inches Top of collar to bottom back- 29 inches So what does this all mean? I directly compared the two because as I said, Mission says you should move up one size compared to the Stratus. The takeaway from the above measurements is that the different between the two is the Proton at XL is about 1 inch wider all the way down, about .5 inches longer, and the sleeves are about 1.25 inches longer, compared to the size L Stratus. Does that mean you need to move up a size? Maybe. The Stratus fits me well but it's definitely very form fitting. The Proton from this review fits me well too, but the fit is slightly relaxed. I'm a 44 inch chest, 195-198 pounds, 6'1" tall with a slightly long midsection. I think it comes down to do you want your shirts, close to skin tight or do you want them to be slightly loose? For me, the answer is sometimes yes, sometimes no. Summing it up: If you're a 44-46 inch chested person and you like your running shirts to fit well but not be skin tight, yes, I would size up one and go with the XL like I did. While my Stratus in a L definitely fits me and I do like to use it in many situations, I have to admit the slightly longer sleeves, slightly relaxed chest and .5 inches of extra length of the Proton is very nice. Also, to be frank, I would also suggest moving up one size in the Stratus as well, even if you are my measurements or close because I suspect an XL in a Stratus would probably be a 23 inch pit to pit measurement, or .5 inches larger in the chest than an XL Proton. In other words, I strongly suspect if you directly compared a size large Stratus to a large Proton and did the same with both as an XL, you'd come away with the Stratus being .5 inches smaller in width, slightly relaxed sleeves and .25-.5 inches longer. Very slight but meaningful differences. I hope all of this doesn't come off as jumbled, I just suspect others may have the same question I did: Does a Proton run small compared to a Stratus? The answer is absolutely, but not by much. * Bonus, if you're my measurements and looking at the Alpha, buy the XL instead of the large if you like a relaxed fit and ESPECIALLY if your midsection is on the longer side. With all of this said, if you've stuck with me this long, again, I absolutely feel comfortable recommending this for running or any aerobic activity, just watch the sizing! Cheers!
