The Google play store, and other app stores, only list the main application (APK) size, not the total app size, which includes the OBB (Opaque Binary Blob) expansion files. OBBs are the extra files that an app downloads to run. There should be no difference in file size before the extra files are downloaded.
From the Play Store Developers section:
The maximum supported size for a single APK is 50MB. If you need to store additional assets (images, for example), you can upload expansion files.
If your app needs more than 50MB of memory, you can use expansion files to store additional APK assets. You can store two expansion files per application. Each expansion file can be up to 2GB in size.
Expansion files are hosted at no additional cost. When possible, Google Play will download expansion files when apps are installed or updated. In some cases, your app will need to download its expansion files.
When you use expansion files, one file is the main file and the other is an optional patch file. Optional patch files are usually used for small updates to the main file.
OBB files use the same compression as APK files do, which is a standard zip compression. APK and OBB files are kept compressed in the file system, and files inside are decompressed into memory as needed, on the fly. So no, the compressed size, which is the space occupied on the device by the APK, is not different from what the Play Store should list.
Finally, the Play store allows multiple APKs per app listing. This is because some bigger apps (mainly games) need to target different resources (graphics engine, display size/resolution, etc.) and they cannot fit that inside of a single APK (due to the size limitation). If you look at the same Play Store page on different device types, you will likely see a different app size listed.