The size reported in App Store app download screen is IPA Size and not App Size.
Measuring the IPA and App size without considering App Thinning is a bit meaningless since the final IPA that will be downloaded will have been processed via App Thinning. The only meaningful way to get an idea of IPA and App size is to archive the app with app thinning enabled for all device variants and check the size report in the generated archive. The steps to create the report are mentioned here. The archive will contain multiple IPAs tailored specifically for individual device variants and also one universal IPA that contains code and resources for all device variants.
The size report has lines like:
App size: 9.8 MB compressed, 29.7 MB uncompressed
For all the IPAs that have been generated. Here, compressed denotes the IPA Size and uncompressed denotes the App Size.
We did the archival exercise mentioned above for two apps
|iOS <12.2||iOS >= 12.2|
|SDK 7.6.0||Thinned IPA Size : 08.8 - 09.8 MB||Thinned IPA Size : 08.8 - 09.8 MB|
|Thinned App Size - 29.7 - 30.1 MB||Thinned App Size - 29.7 - 30.1 MB|
|SDK 7.7.0||Thinned IPA Size - 11.5 - 12.7 MB||Thinned IPA Size - 9.0 MB|
|Thinned App Size - 36.7 - 38.1 MB||Thinned App Size - 30.7 MB|
Above table is the result for thinned IPA and Apps. For Universal IPAs, there is a folder called SwiftSupport/ inside the IPA which adds ~68 MB to IPA size and 144 MB to App size. However this folder gets removed completely in the App Thinning process and has no effect on the final IPA and App size that users download. More Info
Due to App Thinning, figuring out the real size impact due to Swift support in SDK 7.7.0 is not as straightforward as checking the size of the IPA. Due to the additon of SwiftSupport folder inside the Universal IPA, it might look like we are adding a lot more size than we actually are. The real size impact, summarized from the table above is - moving from 7.6.0 to 7.7.0 and above should add ~3 MB to IPA and ~7 MB to app size for iOS < 12.2. Same migration should add ~0.5 MB to IPA and ~1 MB to app size for iOS >= 12.2