I don't have either on hand, right now, but DO have a Glock 38, which uses the same frame -- and it also uses the same recoil spring assembly -- even though the slide is much heftier and physically slightly larger than the 19/23/32 slide.

But, after you asked that question, I checked the Glock site and the Glock 19/23/32 (9mm/.40/.357 SIG) slides seem almost identical. (Cited measurements and weight are the same, although the 9mm slide ought to be a bit heavier, as the opening at the front of the slide is smaller.) The 38 slide is different (bigger and heavier). It may simply be that recoil spring weight isn't that big a deal with some rounds and gun designs.

In an recent discussion here, when an Apex designer talked about the 9mm M&P slide, he mentioned slide stretch/flex seen in the 9mm slide that wasn't observed in the .40/.357/.45 versions. He mentioned one area where the 9mm slide was thinner, and that area was where the stretch/flex was noted (thanks to high speed imagery!) The 9mm slide, at least, was different.

It could be that they ARE the same weight, but have had some areas lightened and others reinforced. (Steel will flex or stretch and then return to it's original -- pre-stressed -- state until it's stretched or flexed too far.)

I know from talking with folks who were familiar with SIG's development of a semi-auto to handle the .357 SIG round, that SIG started with the P-228, but found that the recoil spring needed to make the gun function properly was simply too hard to rack manually. So they reinforced the frame, and added a heavier slide, and created the P-229.

I know from trying a lot of different recoil spring weights with a number of different handguns over the years that it was often hard to tell a difference until I jumped up or down several pounds in spring weight -- and then the difference was mostly noticed when racking the slide manually.

It may be that a single-weight RSA for some guns is simply an ergonomic consideration -- making it easier to use the gun -- and has little to do with gun functionality.
.

I've found over the years that recoil spring weight is important when tuning 1911's for different ammo. Magazine springs are another factor in the mix.