My mini is a study deck -- a miniature Rider Waite Smith. Since I already had a Universal Waite, I got this one to have the "original" colors on hand as well as for the irresistable cute factor. The card stock is pretty much the same as for normal cards. So, while each individual card is shorter and narrower, the whole deck is a normal height -- taller, actually, than some of my full-sized decks. On the plus side, that means the cards are hardy. On the minus side, they're very hard to shuffle riffle-style.
An experience poker player was stumped on how to shuffle them, save for cutting the deck and basically shuffling it in halves.
They are adorable, though. Somehow, the artistic quirks that drove me bonkers in the full-size version are easier to take in the cute version.