Remington introduced the first 20 gauge 1100 "Lightweight" on the 12 gauge sized receiver, with Mahogany furniture. They were looking into a new sized receiver for a lighter, scaled gun a few years later, when someone suggested the 28/410 sized receiver should work fine, because it is forged steel. They tried it, and aside from some issues with the first models - the LW20s - the rest is history. Got that direct from Wayne Leek many moons ago on an airplane ride. BTW, I have never personally seen an LW with issues, but it appears that with extensive use of heavy loads they could crack a receiver. Similar to the Sportsman 58s. The scalloped ejection port and the relieving cut like the 12 gauge 1100s solved things.
I love my LT20. Recently put a straight gripped stock set on it, and had a blast shooting clay pigeons yesterday. I bought mine - a Magnum model - back when you could still shoot lead in a 20, but not a 12, for ducks. I was frankly surprised at how well it worked with 3" loads on the big ducks, and in the years since I probably shoot it more than anything else. Got two 2-3/4" barrels for it. Bought the Skeet barrel the day I got the gun, and had the 28" Full barrel fitted for coke tubes. Great wet weather dove and quail gun, because I can so easily take it completely apart and clean and dry things. I shoot it better than anything else, but I still love SxSs too.