It will use more, but not exponentially more if de-duplication works as well as is claimed. The problem with AppImages is that they don’t include all of the dependencies, making them less reliable. At the expense of storage space, Flatpak bundles everything for reliability.
De-duplication works better the more Flatpak applications you have installed. e.g. de-duplication saves TheEvilSkeleton over 50GB of storage space here: tesk.page/…/response-to-developers-are-lazy-thus-…