Uni's is huge LOL. I didn't get her spayed until she was almost 4 yrs. She did not go into her first heat until about 13 months, and I think I read somewhere that said if your dog doesn't go into heat by 14 months you need to see a vet. I think. When she was in heat her vulva swelled even bigger. She did not have a lot of blood during her heats, but you could def tell. Also her nipples got bigger and stayed that way since her first heat.
However, my foster Chloe, had a litter before but they did not survive and she was spayed during her c-section. Her vulva and nipples are very small like she was spayed before her first heat. So I agree, every dog is different.
A 3 lbs dog can be spayed, if there is no other underlying condition. Sugarmamma's Sugar is barely 3 lbs and she actually is double spayed. Long story.
She did just fine.