Having cut a few calves I can tell ya that size and shape of blade doesn't matter. Some of those Texican guys use a knife called a calf cutter and it looks more like a straight razor than a trapper. However this is one of the few places in life where a knife can be too sharp. Years ago I was into Japenese water stones. This was long before they were cool. I remember polishing up some knife to like 8 or 10,000 grit and just waiting to get to my first branding. Well it doesn't work. A highly polished edge just slides. I tossed it to a buddy who was also down (off his horse doing groundwork) cutting. He tried it on one and tossed it back said "don't like it , cuts funny". So went to work finding the ideal edge for this job and general ranchwork. After years of working on this settled on a 220 grit edge with just the burr polished off. This is ideal for cutting calves, rope, hay string, feed bags etc all the chorse we might use a knife for on a ranch. We estimate that 85% of our customers are working cowboys/buckaroos, ranchers etc. So it was important to get this right. Now every knife (with the exception of specialty knives like kitchen, hunters, skinners, leather knives) that leaves my shop has that edge and all I hear back is good things.
Another side note is the cowboy is more interested in ease of resharpening than edge holding ability. If ya got to do 200 calves before lunch you are gonna need to resharpen, don't matter what steel your blade is made from or how hard. I think that is another reason for the popularity of the trappers as most of em run fairly soft.