I think it is a matter of opinion. I have asked several vets this question, some say no and some say that a dog may loose a little bit of mass due to the fact of not reaching the age were hormones kick in, such as a heat cycle for a female or testosterone for a male. This is about 8-12 months of age. But not to an extreme that anyone would notice like you are discribing. It would be very minimal if any at all. But all agree it is better and healthier for the dog to be spay/nueter early as possible if they are not going to be bred.

I personally have had owner spay or neuter their dogs at all ages, early as 5 months old and have NEVER seen a difference. I think if a dog is going to be lean and look like a boxer, that is just how they are going to look regardless. Here are a few dogs I have produced that have been altered at a young age.

This female was spayed at 5 months.


This male is 1 year old in this picture, he was neutered at 6 months of age.


These two dogs are from different litters but the male was neutered at 8 months and female at 5 months. I don't see any boxer in them...




One more, this male was neutered at 7 months as well. No boxer here either...