Minor differences in screen size aren't likely to affect data usage when you're using dedicated apps that communicate over the internet; I doubt that Facebook scales the resolution of pictures that it sends to devices differently depending on the device. If it identifies as an iOS device it probably sends the same size pictures to any iOS device. That being said... for general web browsing there likely will be a difference between the data consumed by an iPad versus an iPhone versus a desktop. This is because modern web sites use something called responsive design, in which the layout of the web site may appear differently on different devices according to their screen size. And when they change the layout they will often cram in more or bigger ads on an iPad or a desktop as opposed to an iPhone. And again, I think this is mostly a difference between different classes of devices, not within a class of devices. An iPad Pro screen doesn't have a hugely different number of pixels (iPad Air 2 is 2048 x 1536, iPad Pro 10.5" is 2224 x 1668 pixels), so the layout between those two simply wouldn't justify sending very different contents. Bottom line is that yes, there probably will be some difference between using an iPhone versus an iPad, but it is not going to be so great that it is very noticeable.