EveryMac has good information on machine limits - http://www.everymac....city-of-macs.html - too. :-)
J's black MacBook is limited to 6 GB of RAM. Apparently without pre-screening of 4 GB modules, getting 6 GB to work reliably in some machines was a coin-toss - https://discussions....10356325#10356325 There's lots of black magic in this stuff, unfortunately.
204 pin DDR3 modules in laptops later iMacs can take up to 8 Gbit chips, and up to 8 GB modules are available. So I don't think it's hardware address lines limitation. It probably has more to do with earlier OS assumptions of where memory addresses could be moved to, 32-bitness to 64-bitness transitions, etc., etc. That's my guess anyway.
(Who remembers, like you, when 16 MB was huge!)