I'm not completely clear what you are asking, but I'm guessing you mean that the size of the installed app is larger than the size of the download from the Play Store. If so then yes, that's expected. The apk you download is an installer package, and it is used to build the other elements that are needed for the app to run (the .dex files - Dalvik cache, object libraries, etc). In some cases (usually games) this installation may require downloading more material, but usually it's just using the apk to compile the app.
Another thing is that when you use an app it may generate, download or store data, which also uses space. I'd say that that Facebook app hasn't been used much yet, but it will certainly start caching temporary files when you use it. But again, this isn't a problem with the phone, that's how the app will work.
