On the silver stickers you'll notice the -883 and -683 on the 128MB and 256MB respectively. This indicates 8MB or 16MB individual DRAMS and there's the answer. If you can only have 8 chips per side per stick, and you've already filled the 128MB stick with 16x 8MB DRAMs, then obviously you need to get higher density DRAMS in order to make a higher capacity stick. The 256MB stick uses 16x 16MB DRAMS and you can see at a glance that they are physically larger than the older 8MB ones.
FULL STORY @ OVERCLOCKERS (http://www.overclockers.com.au/techstuff/r_km_150_256/)