RX100's RAW Sizes can it be adjusted?

By the way, you can convert your images to DNG with lossy compression (via Adobe DNG converter). Lossy compression produces quality degradation, but if you are interested in 10Mps only, that's don't matter.

Sean McCormack says, that downsampling is also available: http://www.pixiq.com/article/practical-lossy-dng
I didn't have experience with resizing, so can't say anything significant.