When laser printers power up, they need to set aside memory for a bit-mapped page at the highest printer resolution, typically 600 dpi. If the printer can duplex, it will set aside memory for 2 pages. That is where most of the disappearing memory goes. More memory is also set aside for various other things like input buffers, etc.
When sending multi-page faxes the printer will also store those in memory before it dials.
You say you got the message "fax memory full canceling recv." That seems to imply that the memory for incoming faxes is full, perhaps because a lot of faxes were received while the printer was out of paper.