What’s the difference between a polka dot and a circle?

Is it just the size? Is a polka dot just a small circle?

But you could have a polka-dotted planet though, right? That’d be a pretty big polka dot.

Or is it determined by the company that it keeps? Do polka dots have to hang out with other polka dots to exist?

Can a polka dot exist by itself? Or is it then just a dot or a small circle?