Originally Posted by fietsbob

Seat tube OD minus 2x tube wall thickness ( another variable ) gives you the tube ID, the seat tube ID is what remains.. a thick wall tube with a 9/8" OD can have a small diameter seat post ..

A thin wall seat tube of same OD , can take a larger seat post ..
Or a seat post sizing shim sleeve to use a smaller seat post

For example a telescoping seat post (a Brompton option)

has a smaller seatpost inside a larger one and that larger one fits in the frame..
it has 2 QR you open to fold it , so a longer one fits ,when folded, to not protrude
any further than the shorter standard one