It may LOOK smaller, but it ain't
R-91: 78.9x18.0x72mm, 110g, 153g w/batt
R-900: 78.2 x 17.1 x 72.0mm, 94g, 120g w/batt
Size-wise, it's not a very big difference.. but it seems to be quite a bit lighter.
The 21 hr playback and the 8 hour recording time you can get on the internal battery alone is a FORMIDABLE achievement imo (talking about the R-900)..