I actually didn’t know that they did but I looked it up and you’re right. Small dogs breeds love longer than large ones. Apparently it’s the weight not the height that matters. I don’t honestly know but I would say that it is probably something to do with their hearts. Dogs can have quite problematic hearts.
Pity you didn’t ask yesterday coz I was the vet’s last night getting my cat vaccinated and I could have asked her. I’ll ring her and see what she says and get back to you later. I hope that’s ok.