chopping axes tend to be thinner profile to cut across strands, splitting axes a little thicker/wedged to split fibres apart. Lots of sizes of handle length and head weight upto about 4 0r 5lbs typically. the big ones on long handles such as the fiskars x27 (36″ handle and about 4.something lbs head) will split most hand splittable stuff and is light enough to swing for extended periods by most guys.
A maul is like a cross between a sledge hammer and an axe, its heavier and a bit wider (although not much, the wedge angle on my stihl pro 8lb maul is virtually identical to that on my x27, but the extra weight in the head makes it look far larger). Nasty wood to split that just won’t pop with an axe can go with a maul. round size has a little but only a little to do with difficulty. If the grain is straight a 3, or 4, or 6′ round will go if you strike it accurately or whittle off the outside like jimjam above. if the round has wavey grain or knots its harder. Sometimes a maul, which has more oomph, can be better, however unless you are Paul Bunyon, or very fit, you’ll find it hard to swing a heavy maul for long.
wedges and a sledge can work better on some stuff, but not everything. Personally I hate resorting to wedges with a passion, but am forced to sometimes.