I am in no way an "expect" on neutering but will tell you my experiences. When I got my very first Yorkie, I opted not to have her altered. Not that I was planning on breeding, but like you, was worried about the surgery. After her first heat cycle, I called the vet to find out how soon I could get her in! She weighed 5 pound pre-surgery and at the time of her passing several years later, she weighed 5 1/2 pounds - so she did not experience what I would call weight gain from being neutered. Then, we welcomed Gabby into our home and lives when she was 12 weeks old, already weighing in at close to 3 pounds. By the time we had her neutered at 6 months of age, she weighed 5 pounds - so I figured, same case with Katie.....NOT. Gabby is now a plus size model Yorkie
but I wouldn't have her any other way (she weighs in at 16 pounds!). Last year, we rescued Gage when he was 6 months old with every intention of having him neutered. Life happened, hubby had a traumatic injury thanks to Hurricane Rita, so the surgery was put on hold. Gage was not in the best of shape when we got him, neglected, abused, etc. He finally had surgery this past February and weighed close to 7 pounds. He went in a couple weeks ago to the vet for a skin rash and weighs almost 8 pounds. So, I don't know if the 1 pound weight gain he has incurred is due to surgery or he's just eating right. I will say, despite my concerns, I had the surgery done. Gage was marking EVERYTHING! And like you, I just couldn't have that in the house! Once the surgery was over and he was doing fine, he went from being a "hiker" to a "squatter". If you trust your vet, then trust him/her enough to do the surgery. With this surgery, as in all surgeries (even for us humans), there is a risk. But my vet had his assistant call me when surgery started, when surgery was over, and when he woke up (I got similar calls on all 3 of my babies). I think the decision is yours alone to make but I can tell you from my experience, it helped Gage and his marking.
Best of luck!
Suzi