Honestly I presume it.
This because it is supposedly part of her old armor (she uses some scraps of her old armor, so glove and shoulderplate seem the most obvious). Then the glove seems large enough to hold her entire hand without even touching the fingers and has a glowing rune on it. It seems quite reasonable to assumed it too was magical.
From her background:
So exceptional was her passion that the High Command recognized her with a black stone rune sword forged and enchanted with Noxian sorcery.
and from her judgement:
The remnants of her Noxian armor are tarnished, they speak of her absence from duty.
Try to imagine her in a full set of armor as huge as the shoulderplate/glove, that's what she wore before she exiled herself.
That doesn't answer any of the questions I gave you. Sorry if I'm coming off as a asshole, but this has been frustrating me for a while now. If I end up lacking any other reason, I'll just have to settle with what you gave me, but it leaves a bunch of questions unanswered and creates even more.
Also, its worth mentioning that if Riven is artificially using a hand much larger than her actual hand by using a exoskeleton which uses technology, or rather magic, that already exists, then the point of Vi making her trademark Hextech gauntlets are pretty much out the door as well. Vi could have just used what the Noxians apparently already had available in the form of armor if your theory is true.