When updating a world from 1.17 using the "Optimize World" feature, the "Level" Compound in which all chunk data is stored in earlier versions, is not deleted. The effect of this is that world size will increase significantly since everything is duplicated. It will only be deleted when the chunk is loaded.
Attached is a pretty-print of chunk (0,0) of a Void superflat world created in 1.17.1, after optimizing it in 1.18 pre-6 (before opening it)