The smaller size is just standard mass market size. It's pretty common for most books to have a mass market edition; at least, it used to be. The larger size you have is called a "trade paperback" edition, and is typically more expensive than a mass market, but less expensive than a hardback. The trades are often better quality than the mass markets (although, that's not always true).

Generally speaking, I prefer the mass market size, mainly because of the cheaper price; but that's also what I'm more accustomed to, as it used to be that many books would get a mass market edition, but not a lot of them would get a trade printing. Now, it seems, the trade editions are more common, and it's harder to find mass market editions. If you have any used bookstores in your area, though, I'm sure they'll be full of mass market paperbacks.