Well I tested it recently I had to drive somewhere and back twice so for the first journey and back I used my car and for the 2nd journey and back I used hers.
Same driver, same conditions, same traffic, same driving style. I got 23MPG in the Golf and 36MPG in the Polo.
Pretty impressive, its because its only a 1.4 turbo but it has the same 0-60 time because its so much lighter