Many good quality side by side 20's made 1950's to 1980's, you just need to search one out. Somewhere in this period thanks to several prominent gun authorities (writers) such as Jack O'Connor, Townsend Whelen, Elmer Keith and others the North American gun buyer became convinced that a twenty should be chambered for 3" and it needed more weight and bulky stock dimensions to accomodate the increased recoil that this brought. The almost universal acceptance of choke tubes meant that barrels needed to be thicker, again increasing the weight but conveniently also decreasing production costs for the barrels. With many Americans bigger means better and their influence was to try to make a 20 compete with a 12. It can't successfully be both.
A lightweight well balanced responsive 20? Look for an English 20, hard to find and expensive but always below six pounds and often around five and a joy to use. Some of the Spanish boxlock 20's (not the sidelocks which are fine guns but heavier), especially if made for the European or British market are very nice but most of them made for the entry level North American market are slugs.
Many Italian makers large and small made fine lightweight shotguns and many of these are still out there in fine condition.
For a new gun that will fill your requirements look at FAIR, FAUSTI, RIZZINI, they make fine lightweight scaled frame small bores and will even provide fixed choke barrels on special order. Any manufacturer can make a solid sturdy gun inexpensively if weight isn't a consideration, it costs more money to make a lighter gun. Don't expect to find a new fine quality lightweight gun at the lower end of the price scale, if you have a moderate budget you need to look for quality older guns by Bernardelli, Beretta, Sarasqueta and many other European makers.
