hello seeky15,
we can see different file size for A/B partition images, but they’re same with the raw image.
it’s mksparse utility to create a compressed version of the root file system, it’s only a smaller image size to speed-up image flashing.
i.e. system.img.raw → system.img.
since I can also see both A/B partition raw images with the same file size, you may ignore the difference.