Per Logitech's website:
-G903 is 3g heavier, 0.3mm longer, 0.5mm narrower, and 0.4mm taller than G900.
-G703 is 0.2g heavier and the same dimensions as the G403.
I wonder if the "Lightspeed" wireless fixes the skipped polling rates
that we see with the G900 and G403. If not, really the only reason to upgrade to the G903/G703 from the G900/G403 is if you want to use that tiny wireless charging mat.