The vehicle that returns to Earth is Orion which stays in NRHO and does not bring its fuel to the lunar surface.
Return payload constraints are probably from using Orion as the return vehicle. Mass to the surface is much higher than Apollo since that is launched separate from the crew.
I thought the return vehicle was a to-be-developed direct-return vehicle from both SpaceX and Blue Origin (both got contracts, and supposedly both's versions will fly)?
[EDIT] Apparently there are multiple plans involving even more spacecraft, because why not I guess? It's as you describe for Artemis III, but then gets way more complicated with Artemis IV, involving more spacecraft for some reason.
As far as I know all of the known Artemis mission profiles only use the lunar lander to shuttle from NRHO (lunar orbit / gateway station location) to the lunar surface and back. All crew return is planned to be done with Orion for now.
NASA has optioned an additional lander from Blue Origin but that would be taking the same role as SpaceX's lander, shuttling from lunar orbit to the surface and back to lunar orbit.
Return payload constraints are probably from using Orion as the return vehicle. Mass to the surface is much higher than Apollo since that is launched separate from the crew.