In a typical arrangement, the clamp goes around the frame's seat tube and compresses it onto the seat post. So, by now I suspect you've answered your own question, namely that the clamps are bought based on the frame, not the post diameter.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.