It isn’t necessarily a matter of being huge. Even if you’re average, if you’re fairly girthy, it’s probably better for your safety and comfort to wear a larger condom. I used to think Magnums were just for giant horse dicks and dudes who want to show off, and I hated wearing condoms because they were always tight and I had to fiddle with them a bunch to get them on (and often go limp in the process). I tried wearing a larger one and it was a night and day difference. For a lot of guys, it’s not that they have a huge dick, it’s more of a comfort thing.