
I know that with many toy breeds it is recomended to breed a little earlier, at say 18 months, as there seems to be less whelping problems with a young supple bitch.
It is interesting that proportionately tiny breeds have very large puppies for their body size. A toy breed of 4Lb with a 4oz pup, is rather different to one of my breed at 44lb passing a 12 oz puppy!
The large breeds are best left until 3 years to have pups, and the medium usually 2 to 3!