Slow growing is really not a bad thing. How big were Mom and Dad?
I've had some that were on the small side as puppies but ended up well within breed standard when they matured.
Caleb was close to 20 pounds (9 kg) at 9 weeks, but he weighs in today that almost 3 years old at about 79 pounds (~ 36 kg), which is perfect as far as I'm concerned.
Caleb (aka Caleb-Moose)
Ciara(aka Ciara Belle, Black Devil)
RIP Aodhán, Rica, Max, Kelly - gone but never forgotten - forever in my heart