Regarding chain mass, even with big saws, there is a noticeable effect on chain acceleration. I have a carving friend who routinely uses a 16" narrow-tipped bar with his ported and polished Dolmar 7900... running that "sissy" 3/8" lo-pro. With that setup, his saw accelerates like my 346XP - lightning fast. When he puts his 32" bar and 3/8" regular chisel chain on the same saw, though, the saw is much slower in throttle response. He runs his chains just loose enough to be able to easily advance the chain to the next tooth easily by pushing against the raker with his file, so it has to be the mass of the chain that is affecting it's performance... even with a torque monster like his 7900. With a pro saw, you are going from 0 to 70 MPH every time you pull the trigger. F=ma. A 18" loop of 91VXL weighs 204 gr., while an 18" loop of 3/8" regular chain weighs 324 gr. That's a quarter pound more steel you have to accelerate to 70 MPH in a fraction of a second. You just can't accelerate more mass at the same rate without increasing the magnitude of the accelerating force as well. The max power of the saw you are using is fixed, so if you want that instantaneous throttle response, you have to go with lighter chain.
Does any of this matter in the cut? I'd think that the heavier chain having more momentum would allow it to bog down less easily (very slightly less), but the same chain will offer much more resistance in the cut because of the increased kerf width. When it bogs, the response time to bring it up to speed will be slower again.
A lot of guys milling with big saws have switched to 3/8" lo-pro because it saves them time. The chain does not last as long, but I can get Oregon 91VX chain for 15 cents/drive link. That's under $20 for a 32" bar. At that price, even if a chain only lasts for a week, the money they save in increased milling speed will buy several chains.
I don't really have a strong personal opinion on performance differences since time is not a big factor in my cutting. The old standard that many of you grew up on called for saws with low RPMs and high torque. Wide chains that took big bites and held up for a long time ruled the larger classes. They were slow, but almost unstoppable... and they kicked like mules when you pinched the bar in the cut. Today, the emphasis seems to be on achieving power through increased RPMs. With these modern machines, anything that slows the saw down will make it stop cutting. I will say that the world has moved on to using narrow-kerf cutters in all kinds of tools. My cheap narrow-kerf circ saw blades outcut the old 1/8" thick ones by a wide margin. My 1HP homeowner table saw cuts like a 3HP one with a narrow-kerf (1/10") blade mounted in it. My thin abrasive metal-cutting blades cut steel twice as fast as the thick ones (although they wear down three times as fast). So don't be so quick to dismiss the girlie chain, it cuts really well in cases where its use makes sense.
Here's a clip of my friend Mike Page using his 7900 and 3/8" lo-pro to rough out a chainsaw bear in 10 minutes:
