The game uses his own memory management.
There are 2 chunks for the memory, 1 behind the chipcode.prg (even inside as it will use the system 3 logo space) and the 2nd after fastcode.prg which is not very important as it's located in the chipmem.
I just reduced (a lot) the 2nd chunk, when the game wants to allocate memory to 'fast' and it can't, it tries to chip. If there is not enough memory, an error message will be displayed during the game. About 196k can be saved without this error.