A lot of guys seem to use saws in the 70 cc range of pro modles. It also depends on the job. If you never drop trees over 20 in a 50 cc - 60 cc will handle almost anything. I have a Stihl 029 that has droped tons of oak and hickory over the past 15 years. I have only came across a few trees I would not tackle with it. Then again I would not touch those trees with any saw. Just out of my comfort zone.
Hard to believe a 395 is even used at all never mind as popular as the 390.
The 390 retired the 394 and 395 for the Husqvarna lovers that like a bigger saw. They been dust collectors years.
I mainly use a 62cc Oleo Mac 962. For the wood I cut it is usually enough. I have up to 100cc if the need arises but in recent years I have gotten so much wood from tree companies that I hardly do a lot of felling anymore.
My 661 works perfectly for me. But it's not just the saw, bars and chains play a big role to. I run full skip chisel on both of my saws. The first thing I do to a new chain is file the takers down 3 to 5 swipes.