Heavier vehicles would to emit more while smaller vehicles would put out less. While companies like Peugeot and Fiat that build predominantly smaller more efficient vehicles would get no benefit from this, companies like Porsche, whose current average is about 280g/km, would be largely off the hook. German carmakers like BMW, Porsche and Mercedes would have little incentive to reduce the weight of their vehicles if these proposals are implemented.
[Source: Automotive News - Sub. req'd]