How small/big is enough for a harness vs. a collar?

I have a 4 pound dog (papillon poodle) and a 9-12 pound dog (chihuahua) and they’re both a year old. I know small dogs are supposed to wear harnesses instead of leashes when I walk them because of their small necks (it can collapse the trachea) but how big is right for a collar? I know the pappy-poo should wear a harness because he’s so light but I wonder about my chi. He’s very very muscular and has a very thick next (he almost looks like a rat terrier, especially because he’s brown and white) so I wonder if he’s big enough for a collar. It’s hard to find a small-dog harness that will hold him without being super tight that’s what made me wonder if he should be in a collar instead.
what is this “choke” and “pinch” collar thing i keep hearing about? people say it in all my pet questions and i don’t know what it is but i definitely does not sound like something i would subject my babies too.

my dogs 9 , 14 and 28 all wear harnesses.

LED Lighted Pets Leash,Collar, Harness

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