no response from canon regarding rx100

When I say lens size I mean that the max entrance pupil on the RX100 is significantly larger throughout its zoom range than on the S100, and hence should collect more light.

Over their zoom ranges, the RX100 diffraction limited airy disk area ranges from 0.7 to 5.4 pixel areas, while the S100 from 1.5 to 13. The RX100 lens design can thus tolerate slightly more aberration (2x area increase in the DL spot area), which is probably why it's a little more aggressively compact design.

That said, bad glass is bad glass - production yields at high quality seem to be a continual challenge for these ultra-compact cameras.