What is your rationale? Are you saying that it would be better for those pets if we slaughered them after only a fraction of their natural lifespan (like the animals you have no such sympathy for) so they never encounter their genetic limitations?!

Chickens suffer the same sort of negative consequences of overbreeding, but to a degree orders of magnitude more severe. Why is it worse that a pug cannot breathe than that a chicken's bones cannot even support its own weight?

I suspect that the relevant difference is that you abuse chickens and wish to continue abusing vulnerable individuals who are chickens, but you've made the decision to stop abusing pugs, and so feel free to be critical about their treatment. Not to be unkind to you; that is just basic human nature.