It could be argued the BMW X6 is gluttonous. It gets horrible fuel economy, and it's characteristics of size + performance lend credibility to the idea of it being a burden on the road for others.
Finally, I disagree with your general sentiment against criticism. Criticism makes us better.
One thing to think about is that the effect of a vehicle on the environment depends not just on the mpg, but on how far you drive it, and how many people you transport on average.
Crossover SUVs are essentially antisocial - they confer no practical benefit to the driver over a sedan or a wagon, but create considerable additional externalities.
To me, it seems reasonable to question whether an inch more or less does in fact make a "considerable" difference or whether this is another moral condemnation of something trivial.
It's gotten really irritating to me the way people, not just you, continually make remarks that assume quantitative evidence for something, and throw in the word "externalities" as though that proves a point.
Externalities are everywhere, but it's not reasonable to use them as an all purpose short circuit for everything. And they are finite. People treat "externalities" as arbitrarily large, as though that was inherent in what they are. That is, conversationally, the purpose the term now serves.