Headcanon that your animagus ages as you get stronger and better at transforming.  

And so when a wizard transforms for the first time they’re just a little baby version of whatever their animagus is.

Sirius as a tiny black puppy (also this fits the name padfoot so much better than the giant dog).  Peter as a fuzzy little baby rat.  JAMES AS A FAWN.  MCGONAGALL AS AN ADORABLE KITTEN.

and Sirius working so hard to get better and stronger because “TINY PUPPIES ARE NOT PUNK ROCK JAMES” and he has to be strong and mature for his Moony.  

But Sirius is a puppy for like a year? and even after James is at least growing his antlers and peter is a full grown rat and SIRIUS IS A PUPPY.  and he can’t wait anymore and they show Remus and SIRIUS IS A PUPPY.  And then he realizes that it’s not because he’s not trying hard enough it’s because his animagus IS LITERALLY A PUPPY.

and it’s not until Azkaban that his animagus becomes the Grim.  It’s not until Azkaban that Sirius loses part of himself.