Those smaller, turbocharged engines are designed to game the fuel efficiency tests. Real world experience has shown that they often burn more fuel than larger naturally-aspirated engines of equal power. They are also likely to break sooner due to higher complexity and tighter tolerances, which isn't good from a total life carbon emissions standpoint.
I own two <1 year old VW Polos - one with a 1.2L TSI(small turbo) "Bluemotion" engine, another with a 1.0L MPI(naturally aspirated) engine. The 1.2TSI is much more efficient on paper, but guess which one gets far better milage in real life? Bingo - the 1.0L MPI unit. I frequently get >50mpg on longer journeys in that car, while the 1.2TSI is more like 35-40mpg.
Sure - but on paper it's better in every regard than the 1.0L, better mileage, better CO2 emissions, and it's the one that gets the "Bluemotion" badge which traditionally was only given to particularly efficient vehicles. Yet the smaller 3 cylinder engine is the one that is far more efficient here.
