The value of MAXMEMORY is coming from the SYS_REQUESTS table.

Each new request gets a 2 MB chunk for it to use. When that runs out, TDV requests more space, sometimes in 512 KB chunks. When TDV processes a SQL statement, it can use a lot of memory. TDV fetches data and sometimes does several calculations on it before the end result is delivered. If TDV detects that it is exceeding the maximum memory available,it can store the data onto the disk, to free up memory.