Interesting post. A ruler will show that the distance from the back of the front wheel to the back of the car is about the same in the Up and the Polo, which explains why VW admit that they have very similar internal space. The Polo boot is not that much bigger, 280L versus 250L, although there might be caveats/fiddling. Clearly VW must have done some clever business with the engine to reduce the car length (radiator at side not front, for example) which is great because it is so easy to park .
If they can get almost as much space from a car that is 50cm shorter, why would they not do something similar with the Polo, and then get more space inside? I am sure I read that the Up floorpan will be shared with other cars, not just the Mii and Citigo.
Regarding plastics, certainly some cars have plastic that feels cheap. The Ford Fiesta has some cheap feeling plastics, perhaps due to using thin mouldings in places, although my old style Ford Ka had plastics that felt okay, although the styling was a bit impractical in places.