At 5 months, your Doberman is plenty big. If you read the breed standard for the Doberman Pinscher, you will find that they are supposed to be a medium sized dog. Bitches should be 24"-26" at the withers, and dogs should be 26"-28" at the withers. The dam and sire of your puppy sound like they are oversized judging by their weight. As an example, my almost 18 month old bitch is 25" at the withers (the highest point of their shoulder before you go up the neck), and weighs approx 62 lbs. She is a standard sized bitch and may fill out a little more. My old bitch was also 25", but weighed 80lbs - she was just plain FAT - LOL!
It has been very popular to breed oversized Dobermans, however, this trend has resulted in many health problems - the larger the dog, the more joint problems they will have. Reputable breeders breed to the standard - anything larger is a fault.
Enjoy your Dobe - no matter how big or small they end up!