A totally agree that breeding practices and husbandry can improve any line, including size. What I was saying is when a Platy ends up several times the size it would naturally reach, it's probably made possible by Swordtail genes, since Swordtails can naturally reach a much larger size. Every species maxes out in size at some point, no matter how well they're cared for.
For example, you can selectively breed a Neon Tetra for size but at some point they just won't get any larger... although it would be cool to see a Neon Tetra the size of a salmon...LOL. The same goes for the Platy, but mix in a little of the larger X. helleri and their size potential is drastically increased. It's just one factor that may explain the GIANT Platy specimens.
It's just my opinion, but I think it's more "How they're reared than Swordtail linage". Otherwise why aren't commercial Fancy Swordtails large???
The line of Bleeding Heart Platy which the late Derek Lambert brought over from England to give to me were very small (mature males one inch or less) I passed some of these to Randy Meyer and a couple of years later he brought some (same line) when visited me and the males were 2 inches easily and the females 2 1/2- 3 inches!
Also, I had a strain of large Red Swordtails and passed some to a non-Livebearer hobbyist friend and he had 1 inch mature males reared in a 55 gallon tank.