App size has been a controversial topic for a while, because we've noticed a trend in which apps keep growing as time goes by. Larger apps are not only bad because they take up a lot of space on your device after installation, but they also consume a lot of bandwidth for downloads and updates and there is a limit of 150 MB that can be downloaded via mobile.
Shortly after iOS 12 was released, some users noticed very large app sizes in the App Store. If you view the Facebook app as an exe, you will see that the App Store reports the size of the app as 519 MB. If you actually install the app and look at the storage settings, you notice that it costs 316MB on your device. This size varies from device to device, but that is more than 200 MB less than what is reported in the App Store.
The App Store uses a process called app dinning to download only the code and resources needed to run the app to your device. When developers submit their apps to Apple, different packages are created for each supported device. These packages are usually a lot smaller than the universal package, which contains all codes and resources for all supported devices.
We were able to confirm that the App Store is reporting the universal package size for some apps, instead of showing the size of the package for the device you are currently using. That is why Facebook (and some other apps) shows a size that is larger than what was actually downloaded to your device.
This bug has been reported to Apple, we will update it with new information that we get.