Does Apple make a desktop that is somehow "bigger" than the M3 Max?
No, the desktop with the fastest SOC is the Mac Studio, which is still in the M2 series.
Rumors are that it will skip the M3 SOCs and go to M4 but maybe not until mid 2025. They will first rev the MacBook Pros to M4, because that is where there volumes are.
Mac desktops have often used mobile CPUs, like the Intel Core Duo. Main exceptions have been the Mac Pros but those are updated very infrequently. With the advent of the Mac Studio a couple of years ago, they started making M series Ultra chips, which aren't available on any of their laptops.
In any event, the M series Max and Ultra SOCs aren't expected to competed against high end discrete GPU cards.