Small Characters; Heavy Shields & Tower Shields

Hi,
I was just wondering if a small character (i.e. gnome or halfling) can wield a Heavy (formerly Large) shield. To take this one step further, would they be able to use a Tower shield? Are there any differences? Would small creatures be prohibited from using these type of shields? I would suspect the way it would work is that these shields would weigh only 75% of the weight listed and be "sized" according to the size of the creature (but the protection would be to the same proportion as if a medium creature used a heavy/tower shield and thus the same bonuses). Or is it that these shields are just to large to be used by smaller creatures (there isn't anything in the PHB to indicate something like this). Any input on this is greatly appreciated! Thanks.

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