What John said.
A 32 MB 72 pin (32 bit wide) SIMM is two banks of 16 MB each. In order to address 16 MB which is 4 bytes wide (32 bits) the Mac must address 16M/4 addresses = 4M addresses. This requires 22 bits of address. The address is multiplexed into two parts, a row and a column. So your Mac supports 11 X 11 addressing. An 11 bit row address is sent to the memory chips, then an 11 bit column address and thence, some address amongst the 4M is found and 4 bytes are obtained from (or written to) that address.
And 11 X 11 is the most that that Mac model knows how to address.
A 64 MB SIMM is one bank of 12 X 12, or 24 bits of address which gives it 16M addresses with 4 bytes per address. But your Mac only knows how to address 22 of those bits, causing a "loss" of 2 bits or a divisor of 4.
A 128 MB SIMM would be seen as 32 MB, because the 128 MB SIMMs are 2 banks of 64MB, just like two 64MB SIMMs pasted together.