Poids d'un fichier pdf

My guess (it's nearly impossible to say without seeing the actual file) is that those pages share a content stream of some kind, such as an image or font, and there's one copy of it in the entire file, with references to that copy in all the pages the use this resource. When you removed pages it broke those references (for some reason) and that stream was then duplicated, so there's a copy of it for each page in the file. Hence the dramatic increase in file-size. When you added pages it didn't break that link, so no new copies were created and the file size remained pretty much the same.