There are tons of articles on the benefits of waiting to neuter. Looks and size have
very little to do with it! The muscle they
retain from waiting isn't "extra" it is
normal. Because we think of neutered dogs as the norm, people, even vets, say that leaving a dog intact
increases muscle. No! Neutering
decreases the muscling they are
supposed to have. Remember, neutering is unnatural and it
takes away from what the dog should be. Removing hormone-producing organs has a profound effect on the body, especially during the growth phase.
From Whole Dog Journal: "
In 2005, Dr. Zink first published an article, “Early Spay-Neuter Considerations for the Canine Athlete,” which lists studies that highlight the risks of early spay/neuter. One orthopedic issue she mentions (and one that I have seen time and again) is the elongated “look” that results from prematurely shutting off the sex hormones that govern the closing of the growth plates. These longer, lighter limbs, and narrow chests and skulls aren’t just a cosmetic concern: A 2002 study published in Cancer Epidemiology, Biomarkers & Prevention showed that this lengthening of the long bones creates a significantly higher risk of osteosarcoma, or bone cancer, in dogs altered at younger than one year." I would urge you to read this article as well as others you find online via actual studies conducted on the matter.
I neutered both of my current boys at two years of age, give or take a month. I would not recommend neutering before 18 months. If I wasn't living in an urban environment, wasn't social with my dogs or didn't do competitive dog sports, I may not have neutered at all. However, I would change my tune when it comes to dogs in shelters, dogs with genetic diseases or dog owners who cannot seem to be able to contain their pets or who even mention breeding one of their dogs. There are special considerations. If someone feels they MUST take their dog to doggie daycare (though more and more doggie daycare owners are beginning to take intact MALES on a case by case basis) or dog parks, they may want to neuter earlier while still waiting as long as possible. I'm not a dog park person but never had an issue at the type of places where you keep on moving versus standing static (i.e., hiking, off-leash trails, beaches). I would also tell a person who is going to wait and has a male dog to really try to work with the dog on sniffing and leg lifting early on and not wait until it becomes a problem. One of my dogs never once lifted his leg and I swear he was asexual! The other one started the sniffing and leg lifting (never inside) when he was nine months. It wasn't a problem because I never let it become one, but it did annoy me. He also felt more stress during classes when there were intact females as he really wanted to be good but was distracted. As I said, both were neutered at two years old. The one I like to call asexual didn't change a bit but the other one did. He stopped all unneutered male dog behaviors within eight weeks with some stopping in two days and his attitude in class is more focused and much happier. Neither of my dogs ever humped (though most of my friends with spayed females are serial humpers and have spent much time chasing my dogs around and humping them -- go figure!).
Anyway, I would not worry about your dog's size and I would definitely NOT neuter a dog early to keep him within a certain weight. The weight aspect of neutering versus not neutering is extremely minimal and, frankly, unimportant.