A bug in the iOS App Store is making apps look a lot larger in size then they really are.
For a while, many apps have been getting larger in size, meaning they will take up a lot of space on your device, and will also consume a large amount of bandwidth when updating.
Well, soon after the release of iOS 12, many users have been seeing larger-than-usual app sizes in the App Store. A good example is Facebook - the App Store reports the app is 519MB in size, but actually after downloading, it takes up to 316MB (depending on your device). That's more than 200MB less than what the App Store page is reporting.
The App Store does something called thinning to only download code and resources that are required for the app to run on your device. Different packages are created for each supported device (e.g every iPhone model) - those are smaller than the universal package containing all code and resources.
The App Store is reporting the universal package size of the app rather than the package for your device. This is making many apps seem much larger than they actually are.