What some forget is that someone doing tree service work will be into a lot different things than cutting firewood. For instance, Saturday the boys and I were doing a job that involved clearing along a driveway and a lot of it was hedge. (Privet) This varied in size from 1/4" to 5" so we were working it over with machetes, loppers and a Stihl 018C, #1 son broke the chain on that so we went to an ancient Homelite XL2.
Anyway, a firewood cutter wouldn't be messing with cutting this hedge down level with the ground but that's what the customer wanted so that's what she got. So, in my opinion, a saw such as a Stihl 026 or a Husky 346 would be adequate for most firewood cutters. Sure a bigger saw would be nice but how much big wood needs to be cut?
The tree service needs to have a saw to match every potential situation so my take would be: top handle climbing saw, small limbing saw, 50cc class saw, 60-70cc class saw and a big guy for the occasional need (but when you need it you need it) Also a pole saw (manual and powered) and the assortment of loppers, pruners, machetes, mauls, and pretty soon there's no room in the truck for your lunch box...