My personal experience is that small dogs tend to be absolute monsters far more often than big dogs. That's not because they're less trainable but more just because they're small enough that many bad behaviors are ignorable so bad owners don't bother to correct them. A lot of the people I know who have small dogs couldn't train a dog to save their life so they got a dog small enough that they could just call the bad behaviors cute rather than actually doing something about them. If a big dog gets agressive towards every other dog they meet, you kind of have to fix that issue very quickly or completely stop taking them on walks in public. But with small dogs, a lot of people seem perfectly content to let them be agressive towards every other dog they meet and rely on the physically larger dogs being well trained enough to not retaliate.

Of course none of that is to say that you're one of those bad owners. It doesn't sound like you are anyways. But it's for those reasons that I would be perfectly content letting my dog socialize with a larger dog someone was walking but I would sooner cross the street than walk past a smaller dog with them.

No one can tell from walking past how well trained and behaved your dog is or how up to date on its shots it is. It only takes one bad incident to realize there's a lot of terrible dog owners out there and protecting your dog's means not letting them say hi to every single dog they see outside.

I would always ask. You can just say “hey would it be okay if I let my dog say hi?”

Some dogs have baggage or bad behaviors, or maybe it’s also the owner who has some anxiety or social reason for it.

My cute, sweet to me little dog is a murder machine in the streets

It may not be about your dog. I avoid sniffs with one of my dogs because he's so high anxiety. He goes crazy with other dogs barking, growling, you name it. He's never bit anyone but it's just easier to avoid the whole encounter.

Small dogs are too unpredictable and escalate without warning.

My dog would do serious damage if he felt the need to defend us.

Not making any accusations but it's not always about the dogs. Sometimes the owner sketches me out.