Google is about to patch a quirk in Photos that effectively gives iPhone users a free ride. The company told Android Police in a statement that it's planning to fix a Google Photos "bug" that stores iOS photos in their original quality without counting toward Google Drive usage, eliminating the need to pay for more storage when your free 15GB invariably runs out. As the HEIC file format Apple uses to save photos is more efficient than Google's reduced "high quality" JPEG files, Google Photos simply takes the originals -- it'd actually waste space and processing time converting the HEIC images.
You can replicate this behavior with Samsung devices, but HEIC (aka HEIF) isn't the default format and thus isn't used nearly as often.
While it's not clear exactly how Google intends to address this, the solution probably won't thrill iOS users. If Google mandated a conversion to JPEGs for free users, it would chew up more of its own resources and reducing quality for people used to seeing their photos untouched. Effectively, it'd hurt functionality just to ensure that iOS and Android users were on equal footing.