Realize when you're talking about how quickly wood burns or how long it lasts what you really want to focus on is how much heat demand you need for your load and how much you have on hand. The more efficiently your wood burns the more btus you get out of it. Burning well seasoned wood will produce a hotter fire, and one where you don't have to cook the moisture out of the wood before getting a complete burn. If splitting the wood makes it not last as long it could be that you aren't putting the same amount of "weight" into your burner. This would make it seem like you're burning it up quicker. One big unsplit log may weigh more than several small ones split, and your btu's come from the amount of weight of wood you have, not the "volume" or space the wood takes up. That said, split wood may have more surface area and might allow more air to mix with the gases around the wood to burn hotter. Anyway, I really think it's the lesser amount of weight that would make split wood seem to burn quicker than whole round pieces.