Do apps running on iOS 10 take up more storage space than on iOS 12?
Okay, so there might be a couple of reasons for this:


If your iPhone is below iOS 10.3, this difference may stem from the different file system format (Apple moved to APFS starting in iOS 10.3). This optimizes your app code size since APFS optimizes storage out of the box. See Here, Apple's Filesystem Documentation
Otherwise, it might simply because of the difference in handing the iPad and iPhone cases by the developer. There might be a multitude of reasons ranging from internal app data caching, internal testing to A/B testing of features.