Hello Phelype,
How can one rely on the size if the code can advertise the size that is
not the size of the file requested but of another one for which it has
added, on its own, an extension?
As Joseph said, because of a not-too-great design choice in the original TeX (I've had a good deal of trouble already because of the automatic adding of ".tex" in file names).
When the file-related primitives were implemented in pdfTeX, they retained this questionable behaviour. On the one hand, it's a pain to deal with. On the other hand, I think it would be worse if \input behaved one way and other file-related primitives behaved differently. The lesser evil, in my opinion.
Best,
Phelype
(sent from phone)