The CPUs of the ACA500plus and ACA1221lc both only have a total of 16MBytes of address space. From that, you need to deduct Kickstart, IO registers (chipset/CIAs, Zorro space) and Chipram, then you have the total addressable memory.
Since the address space of the ACA1221lc fully overlaps the ACA500plus, the memory on ACA500plus can't be used.
The ACA1221lc already does it's best to fill all the available gaps with memory. It actually has 16Mbytes physical memory, but you can't see it all . The card does support switching to the "invisible" areas in hardware, but to date, there is no sowtware that makes use of this feature. Probably because the only sane thing you can do with it is a RAM disk. You will never be able to make that other memory usable as fastmem.
The only way to get more memory is to use a CPU with full address space, like a 68030, 68040 or 68060 CPU. The ACA1234 comes with 128MBytes of memory, and the ACA1240/1260 will come with 256MBytes of memory.
What's your (memory-eating) application?
Jens