The 361 is what you're thinking about - 36cc's. The 361 is a very strong saw. I was impressed when I used it.
The 390 is literally a miniaturized Echo 500P - even down to the cylinder design. So it isn't as revvy as the 361 or the 2511, but it has a boatload of torque. It pulls like a 50cc saw once it's in the wood. So much so, that with just a few mods like a perforated filter lid, and dual exhaust, I see no reason to get a 50cc saw.
The 390 is a pound less than the 500. The 361/355 another pound and a half less. So, pound for pound, the 361 is an angry saw. My issue with it is the 361 doesn't run 325 as well as I'd like, has tiny fluid capacity, and tiny filler caps, and 10mm clutch cover nuts. So it's more fiddly than I care for, and it really is a pruner. Whereas the 390 is good for a work saw for felling small trees and bucking them - standard sized filler holes and nuts, good fluid capacity, and still weighs nothing with loads of grunt.
361WES on the top of these saws, 390 top right, and dwarfing them both is the 241 in the pic.