If your point is that going on about evolutionary psychology adds to the obfuscation but not to the insensitivity, I disagree.

It could go either way. Digression into a bunch of theory and science impersonalizes things as well as focussing on 'me' instead of 'you' The main problem with getting into a big speel on science is that it increases the total time spent dwelling on the painfully negative topic. The fact that it is talking about the science isn't the insulting part.

There are often ways of more or less sensitively coming clean about (what one takes to be) one's true reasons for breaking up. Maybe you wouldn't go so specific as "you're too fat," but you could talk about lack of physical chemistry or whatever without uttering a falsehood or being too misunderstood.

Talking about 'lack of physical chemistry' is less insulting by virtue of being a vague pre-packaged euphemism rather than brutally personal criticism of highly status-sensitive personal features. It seems to be an entirely different kind of difference to whether you mention evolutionary psychology or not.