First off the smaller diameter sprocket on the .325 setup spins faster. Secondly the number of cutters between the two, doesn't equal the difference between full comp and skip chain. Third consideration is the width of the chains and consequently the width of the kerf.
This is false. At the same engine engine rpm the 3/8 chain will spin faster because the sprocket is larger.
Assume your saw is running at 10,000 RPM in the cut with a 7 pin drive sprocket.
The 0.325 pitch chain will spin 31.6 ft/sec.
0.325"x7x10,000=22,750 inches per min
22,750 in/min ÷ 12 in/ft = 1895 ft per min
1895 ft/min ÷ 60 sec/min = 31.6 ft/sec
The 3/8 pitch chain will spin 36.45 ft/sec.
0.375"x7x10,000=26,250 inches per min
26,250 in/min ÷ 12 in/ft = 2187.5 ft/min
2187.5 ft/min ÷ 60 sec/min = 36.45 ft/sec
So why did I do all that math? Well, proof is in the numbers but it's easy to see that the engine will have to work harder to maintain the same engine rpm because it's doing more work.
As mentioned by others, the 3/8 chain cutter is wider also so there more work there also. Add all that up and put it on an underpowered saw and it could cut like crap.
The way around this is running a less aggressive filed 3/8 chain. You can file your chain to the saws ability and if you take a saws that is a little under powered for 3/8 chain and file it properly it will perform reasonable.