How to compute all space taken up by an app and its associated files (Using Android 6)
While trying to determine the size in memory taken up by a given App I got two widely differing values. 
    Settings. Applications. Application manager. Whatsapp.
gave 134 MB (App 63.90 MB, Data 71.18 MB), while Total Commander gave 900 MB.

How can I get the size taken up by all files that reside in my cell phone associated with a given App, which would otherwise not be there? I am pretty sure it must be closest to what TC gives, but I am not sure everything from WhatsApp is within its directory, and even less sure that it applies for a general App.

Why does Settings give such a low value?
What is it counting as App and Data?



Related questions:

Similar (old) question (but not the same):
KitKat - How to identify the REAL size of apps (app + datas + ...) to be able to install other ones

Related (very old) question: what is my personal data storage as opposed to program storage  and how can I reduce it?



Questions related to the issue that inspired this one (I am constantly running out of space):

Missing space/Storage space running out Galaxy S2 [tried almost every solution I can find!]

Why am I constantly running out of space on my Galaxy S2?

Microsoft apps always eating up space with data, for apparently no reason

I can&#39;t find files taking up space in my android phone
Later versions of Android do not count cache data when showing the app size. So you can f.e. see that Instagram takes 150 Mb of your storage while there are more than 2 Gb of Instagram cached data (true story!)

As for Whatsapp, the size of media sent and recieved via Whatsapp (images, videos etc) is not shown in Settings. But since these media are stored in Whatsapp folder, their size is counted when you look at folder properties in any file manager.