Here are some contenders for the title, depending on your definition:
By Weight:
* Hippopotamus: A newborn hippopotamus can weigh up to 50 pounds, making it one of the heaviest baby mammals.
* Elephant: While not the heaviest at birth, elephant calves gain weight very rapidly, and can reach over 200 pounds within a few months.
By Length:
* Giraffe: A newborn giraffe can be up to 6 feet tall, making it one of the longest baby mammals.
By Other Measurements:
* Blue Whale: While not technically a mammal, the blue whale is the largest animal on Earth, and its calves are born weighing up to 2 tons and measuring up to 25 feet long.
Ultimately, there's no single definitive answer to the question of the "largest" baby mammal. It depends on how you define "largest."