I've seen way to many trees that went the wrong way because people weren't using a large enough saw for the job and it couldn't spin the chain fast enough to cut the tree. I have yet to see anyone who can safely control a cut with a saw that will not reach all the way across what they are cutting otherwise no one would have a bar over 28 inches. I personally have bars all the way to 36 right now and when the extra money hits will have either an 088 or a 3120 with at least a 48 inch bar seeing as most of the normal pines down here are 30 inch or larger at the base. The water oaks tend to be even bigger.
My ms880 pretty much stays in my tool box with no bar on it until the wood gets pretty dang big, over 4 feet.
My 394 sees a little more action, between 3-4 feet.
It's my 365's that get used the most up top when necessary.
And on those occassions when I want to buck up some serious wood on the ground, my ms440 with a 24 inch bar gets fired up.
If you need a bar that matches the wood's diameter, you must be a rookie that can't cut straight.
The majority of arborists I know, even those that have been at it for over 10 years, can't sharpen chains, or dress their bars properly, just enough to get crookedly by with. I see their chains and laugh as they get loose then tight as I turn their chain by hand. When I suggest new drive washers to cure that problem, it seems to go in one ear and out the other.
I have a 262xp with 325 pitch chain that I like to use in large wood in the tree, particularly if there's alot of it up there, not quite the power of a 365, but considerably lighter for a wimpy old foggy like me to handle.
jomoco