The unlimited photo backup resolution for Google Drive is set at 16 megapixels. Only anything above 16 megapixels will count towards your normal quota. 16 megapixels is ample for most smartphone users.
Do you have a source for the 16 megapixel allowable? Because according to this Google support page, unlimited photo storage only applies to photos with a resolution of 2048 pixels on the longest edge, which would mean a 4 megapixel limit (2048 X 2048).
https://support.google.com/drive/answer/6093613?hl=en