is simply the volume of water stored in the crop of the female at left. She had been there for a longer time and was ready to take off (rather heavily, I presume). By contrast, individual at right had just "landed" (so far this can be said about a water surface) and had not (yet) swallowed any noticeable quantity of water.
As ron states below, more info is available under his linked picture of Polistes aurifer.

What do you mean crop? I have a P dominula colony and I too never see them swell up. Infact I never see her at the water dish either, though she eats well. (honey and hornworms)

Have the luck to live in a thermically controlled environment. So they don't need to take water in order to cool off their nest. But wild colonies often have to do that intensively by dry, hot weather, especially those ones under roof tiles.
If you stand near any kind of basin this summer, you will be able to watch the "swelling up" phenomenon. It takes an astonishly short time (maybe 20 seconds) for an individual to nearly double the volume of her gaster. That said, I admit the female at left is an example of an especially zealous water carrier.

I have gotten the impression that the black may actually becomes a more brownish color after a wasp fills up on water. I'm not sure at this point, but it could be so.

Thanks for clarifying; I've never seen a Polistes take in that much water so that the metasoma stretches to that extent.

I wish I had more time to watch them today. I just looked in on them and saw a lot of variation even beyond the range of variation that is shown here. The sort of thing that keeps me guessing.

Happy to help! P. dominula is a pretty unique species in many aspects.

You might see a few males as well which might explain some of the variation (finding males can be a little unusual for this time of year, they usually emerge near Fall). Unmated worker females may sneak an egg of their own into a cell and it may reach adulthood if the dominant queen doesn't discover it. These turn into males, and have 13 antennal segments and 7 metasomal (abdominal) segments (females have 12 segments and 6 segments). Or you can identify them by sight by their long curly antennae and yellow, squarelike faces.

Some female workers will also be noticeably more smaller than others. These are most likely the first brood of a new nest. They are smaller because the queen didn't provide them with enough nourishment.

You got me interested in this as well, I will do a bit of digging and see if I can find anything else on their size variation.