From the dogs I have been aware off it certainly does seem to be the case!
There is a lot more strain on the heart of a Great Dane say then of a yorkshire Terrier!

However - there will allways be exceptions!
I do think a healthy diet and execise regime can prolong the life of any dog! and when I did change my insurance the other day the guy on the phone did actually say 'dogs are living a lot longer now!

regards
DT