The RAM capacity of any computer is limited by the memory controller chip on the motherboard. It can count up to a certain number of bits on a memory chip, and no higher *.
Granted that when a computer is first released, the specifications may not allow for the maximum module size that the memory controller is capable of addressing, (perhaps because the modules aren't on the market yet) which is why a few machines like the PB 12" 867 and the eMac 1.25 can take 1 Gb modules when the original spec said 512 Mb.
But in most cases the limit is hard-edged and can't be patched without changing the motherboard entirely.
*: This is one reason why some modules work and some don't, for example 256 Mb in a beige G3. 8-chip modules will never work, because the individual chips on the module are too dense, the memory controller can't "count that high". But a properly designed 16-chip module can work, because the chips are lower density, and the controller can cope with them.
Thanks
Trevor
CanadaRAM.com