"Wait, that didn't work right" - the Dysfunctional Rules Collection

[flumphy]

However, there are no rules for Arcane Spell Failure for holding in your hand any large and bulky item which is not a shield. You can cast spells just fine while holding a dumbbell, or a ladder, or a sack of treasure, for example. A buckler? Heaven forbid, no.

Animated shields also contribute to ASF, so it pretty explicity doesn't have to do with you holding anything. I've always just ripped off Runescape's explanation that the bulk of the armor itself traps magical energies. It makes as much sense as anything.