I think people who are into crafts. They have all of these yarns, construction papers, various tools and stuff. All so that they can say that they have all of these projects in mind that they want to do. But they never do them so they get more crafting stuff and it just eats away storage until their place is practically consumed by it.
Crafters are definitely up there, overall - but I think wargamers might beat them. Hundreds to thousands of models, paints, brushes, terrain, carrying cases, books - it adds up to a hoard of epic proportions. That’s just personal experience though. Lego fans can also get to be out there, and TCG players.
Gotta second the card gamers. I have no idea what cards are in my collection anymore, and i only have three longboxes of cards. I’ve seen far bigger collections. There’s a few reasons a quit that hobby, and this is one of them.