Quick googling:

https://europe.autonews.com/sales-segment/automakers-fight-r...

> "New CO2 rules will require automakers to fit thousands of euros of tech to each car," Max Warburton, an analyst at research and brokerage firm Sanford C. Bernstein wrote in an April report. "Big cars have the price points and margins to cover these costs. Small cars simply do not. These segments may soon be abandoned by many manufacturers."

Oh, so it's about inexpensive/expensive cars, and size is just something that correlates with that. I see.