Doesnt it just fit? What is the issue?
What they mean is if one were to RP a 7'3" Argonian trying to squeeze into armor made for even a 6' tall Nord. Yes, in GAME terms all the armor just "fits". In RP terms, if one wanted to go to that extreme, a 7'3" tall bipedal reptile would have issues wearing armor made for almost anyone else without a lot of fitting. That ignores that fact that a large bipedial reptile might not be quite the same shape overall as a large human like well, a Nord.
Or if that is confusing, imagine a lot of modern basketball players trying to buy clothing off the rack in a store. Most need at least some of their clothing custom tailored. Now think about the guy's comment.