Although the quote continues on to make an exception for weapons and armor, I have also ruled that magical full plates refit to wearer. This is why non-magical full plates are rare (and non-masterwork are nonexistent) in my world. Anyone likely to sell the armor will acquire either a magical full plate or only a half plate.
Actually the quote goes on to support auto fitting for magical armor, not an exception.
Only say "it doesn't fit" if there is a good reason.
It then goes on to give examples of "good reasons" like self absorbed drow-elves or dwarves. But it is clear that as a general rule magic item size should not be an issue with treasure.
The use of the different "size" categories supports an assumption that any non-abnormal shaped creature of that size category should easily be able to pick up and use the item.