Note that on larger wood, the lower penetration of the longer blade is not always a detriment. Realize that while the penetration is reduced in depth, it is increased in width over the axe.
Thus for example if you are cutting six inch pine with a BR, you make one cut per side on the v-notch and get say 1" of penetration, with a decent hatchet you get 2" per hit, but it takes four hits as each hit only makes a three inch long cut. In four hits with the BR you get two inches of penetration as well and thus both ideally chop at the same speed.
There are a few complications however, first off all it is somewhat easier to align up axe hits across than it is to place the knife hits in the same place, as the axe hits don't need to be quite as precise, but if the blade hits are even a small distance apart the penetration doesn't add up, you just get what is called a stepping pattern.
Secondly, the inherent lower penetration makes the axe likely to be more fluid in the wood, and have a greater tendancy to break the wood clear, thus it can work with bigger notches. How much of this is a factor depend on the nature of the wood.
On soft woods like pine, a stick would have to be massive before a BR would not be efficent, you are looking at trees ~300 lbs. Most shelter/fire sized wood is 3-4" thick and only 75-100 lbs.
Remember that wood weight goes as thickness cubed, so if you double the width, the weight almost goes up by a factor of 10, so trees get difficult to move fast.
Where hatchets come in well is working on really dense wood, and/or knotty wood. I would not mind chewing through a 6" pine with a BR, but would want a hatchet on spruce, especially if knotty. If you have to work on large trees with large blades then work on angles so as to reduce the profile of the blade presented to the wood. So when bucking, lean slightly over the log and v-notch the far side, then move back a little then v-notch the front side, then v-notch out the middle.
-Cliff