Why the Canon S90 does not handle macros as well as the Canon G11? I understand the S90 has a minimum focus distance of 5cm, while the G11 can focus at 1cm, but why there is this difference since they are so similar otherwise?
The lens on the S90 is optimized for compactness and letting more light in, so it sacrifices closer focus capability compared to the larger and slower (larger f number, smaller aperture) lens of the G11. "Macro" lenses for SLRs are bigger and slower than the corresponding non-macro lenses, too.
The sensor affects minimum focus distance just because it is easier to make closer focusing lenses if the sensor is smaller, even though the lenses are not really optimized for that. Macro lenses for SLRs cannot even focus as close as the S90's 5cm!