Dog "bully" - dog smells at another dog's rear

Last night when I was talking a walk, a guy was with his small dog talking a walk too. Ahead of us there were two large dogs laying on the ground resting. When the walking dude and his dog passed the other two dogs, one of the larger dog suddenly jumped up and chased the smaller dog all over the place. When he caught up with the smaller dog (less than 1/5 of his size), the larger dog put his nose against the smaller dog's rear.

I am not exactly sure the gender of those dogs, because it was quite dark. They are different breeds for sure. The smaller dog had white fur, the larger dog had black fur.

What was he doing?

I'm not sure about the chasing part as it seems aggressive or territorial. But, the rear smelling is normal dog behavior. I think this is how they greet each other and figure each other out. It is sort of like when humans ask each other their name, where they you from, what type of work do you do. They pick up so much information from scent.

My sister's dog stays with me sometimes. I will take my dog for a walk to give her dog a break. When we return, my sister's dog is all over my dog smelling her rear end. I suspect she is getting all sorts of information about where we went from that sniffing.

So it wasn't an aggressive behavior? The smaller dog was running away when it was chased by the bigger one. Of course the bigger one ran faster due to much larger size and longer legs, so he caught up and put the nose against the rear end of the smaller dog.

It's partly aggressive behaviour because the little dog obviously didn't want it's behind to be smelled by the other dogs. Unless it just ran away playfully as some dogs would? It depends on the situation and I can't tell without having been there.

Anyhow like the other posters commented it is perfectly natural for dogs to sniff each others behinds as it's a way of identifying one another and in some cases greeting each other. Normally it's totally harmless.