Are you asking if they get chubby or hit a growth spurt and grow bigger? Bentley was neutered at 5 months old and was still growing now he is on the thin side at 7 pounds, he's tall and thin. Cha Cha (my rescued chihuahua) was neutered at 2 years old and she's normal weight at 4 pounds. Sherman was neutered at 12 weeks of age and now he's 8 months old and still has that puppy chubbiness weighing 3 3/4 lbs and he
loves to eat! So I may have to watch that he doesn't get too chubby.
As for personality changes, Bentley did calm down since he was a pretty hyper puppy, but he's a still a very active 3 year old today. ChaCha didn't change at all but she was over a year old when she came to us so I don't know how she was as a puppy. Sherman is active but not hyperactive.
I hope this helps!