What I don't understand is why people think they need to chop down a tree in their survival scenario. What else is that knife going to do better for you than a smaller one? Ever skinned a deer with a 15 inch bowie? Most of your shelter is going to be made with fallen branches, trees and easily accessible foliage. Going full-bore and chopping down trees your first day to try and make a shelter just isn't practical, you're going to be gathering felled materials. Unless you expect to spend weeks or months in the field and want to build a permanent structure, I can't see any need for this ability.
If you know you'll need to cut large limbs and small trees then a hatchet (at least) is in order, if you don't know what to expect, why bring a knife that could limit you? A really large knife is just not convenient for day to day tasks, you may be using your knife to cut food, whittle, cut rope, clean animals, pick splinters etc. Anything over 4" isn't necessary for nearly all of your tasks and a could be a hindrance in many.
If I needed to bring a chopper or bush knife, I would try to go with something light and fast like Scott suggests, but I'd still take a small knife over it unless there was a specific scenario or terrain that justified a large blade. Take your queues from people who spend their time out there.
There's a reason Les always have a leatherman, there's also a reason so many backpackers go to so much effort to reduce the weight of every item they carry. I've known many folks who only carry a slipjoint trapper hunting - it's enough knife to do most anything you need in the woods. It's also a lot quicker and easier to maintain a small edge. My first hunting and camping knife was a Swiss Army knife. There's a reason MacGuyver always carried one of them too