Lets see the Polo is substantially smaller than a Prius in usable space, and when equipped with an A/C it gets UK MPG 70.2 with a 5 speed manual.
Unfortunately for a real commute car a stick shift makes very little sense, so with an auto the consumption would be?
In the UK no A/C might be OK, but most places it will not fly.
I wish I could get the interior dimensions, but this will give folks an idea (all dimension in meters)
2.466 |3.916 |1.650 |1.467 |Polo 5dr
2.700 |4.450 |1.725 |1.490 |Prius
2.578 |4.554 |1.781 |1.459 |Jetta
Now US Interior dimensions for Jetta and Prius (no Polo, yet??)
Cabin|Trunk (Both in cubic feet)
91 |16 |Jetta
96 |16 |Prius
So suddenly it paints a very different picture. A much smaller car, with a manual transmission (which won't fly in US metro commutes) that is almost 2 seconds slower to 62 miles per hour gets 6.8% better gas mileage (if equipped with A/C).
And that was UK MPG.