Completely different chips; the VRAM differences are from how GDDR can be used, with either 1 or 2 chips on a single 32bit bus, the configuration with 2 chips is called clamshell. The 7800 XT and 7600 XT have same VRAM but the 7800 XT has a 256 bit memory bus while the 7600 XT has a 128 bit memory bus. Meanwhile the 7700 XT with 12 GB is on a 192 bit memory bus.

The workstation edition of GPUs usually do the clamshell configuration so they can easily double the VRAM and ramp up the price by a couple thousand