Got to be the combination of low weight + very low aerod drag, and as well all know :wink:
small engines running unthrottled at full torque are producing their best specific consumptions (as a broad approximation to the facts)
Whether this will really take off or not is the question... :?: peeps are still buying big SUVs and 4x4s here in the UK - just for running the kids to school - a sort of over-protectionism gone mad, together with status and I have to admit, ease of getting in and out of them.
Mind you, these and equivalents from other marques have become very popular too over here...
http://www.vauxhall.co.uk/vx/vxfilter/vxfilter.do?method=getBodyStylesListDinky little things that sit upright, but are small and give great mpg