I believe (though I dont have my micrometer) the gp100 and 686 cylinder are near the same diameter. Ruger just prefers strength over capacity in their wheel guns.
Smith introduced its Plus in 1996.
Ruger has had 20 years to make a decision.
The GP 100 seems to hold its own in sales without the extra round.
I have always surmised, although I certainly am not an engineer, that the cast steel is weaker then the forged steel of the Smith in an equal amount for an equal amount and therefore removing more material from the cylinder may not necessarily compromise its safety but possibly make it weaker then they are known for.