Why do the system apps show 0B memory space occupied? I had noticed in my phone's applications manager, that the size of the system apps are displayed as 0B, which indicates that no memory space is occupied by that particular app.
Please see this screenshot in which the pre-installed Browser app is shown to have occupied no memory space:



But, I know that every app is composed of several files, so no app can have 0B size. But, the size of third-party apps are shown correctly.
Why does this happen ?
Many system apps have their (original) apk's stored on the system partition, which as it is fixed in size wouldn't really count towards available storage.  Nor are they user removable.  So it doesn't make a lot of sense to show a size for them, when they are not consuming any fungible resources which a user could chose to put to a different use instead.

And if pre-dexed as is typical for things packaged in a system build, they don't need those lookup resolutions against the installed system libraries stored in a runtime-writeable place either, something that is ordinarily a cost of having a latent downloaded or sideloaded app installed.


  When the system apps are used, they create some data and cache, for which their size becomes a few kilobytes only


Indeed, that makes sense, files created during use cannot be stored on the system partition, but only on the data or cache partitions, competing with what other apps need to store there.

All of that is not to say that some other configuration might not report this differently, or that some other system apps may consume fungible resources by the time the user can interact with the system, but only that what is being reported is an arguably legitimate representation of reality.