Sooo …there are two possible explanations:
Either Link is somehow able to magically resize everything he owns so it fits him perfectly. This would also explain why oversized weapons like Lynel Crushers suddenly shrink to a convenient size once Link picks them up, but if it applies to armor as well, then it should not have been such a big deal for Muzu that the Zora Armor “fits Link perfectly”. If the armor resizing thing is canon lore-wise, then it would have ruined an important key moment in one of the main quests.
… but if the armor resizing ability is NOT canon, then why did the Warm Doublet fit Link at all? I guess the “Old Man” made the Warm Doublet specifically for Link and just pretended it was his own shirt, hoping that the little guy doesn’t ask too many questions.
… or the old man once wasn’t a big man, but a small boy, and it’s his old shirt from those days?
But why did he keep it? Sentimental?
You never looked at some item in your possession that you actually haven’t even touched in 5+ years and thought to yourself “but what if I somehow, someday, need it again for something?” …and then kept it for another 10 years instead of throwing it away? =P
Naw, bro, I’m not a hoarder or a video game character, lol.