The filesize in dictionary.tables (or sashelp.vtable, which is a view to the same dynamic resource) is in bytes; the output of my UNIX command is in Kbytes, because of the -k commandline parameter.
Without the -k, du would report disk blocks, which is not that informative, as it may be dependent on individual filesystem parameters.
If you want to make both outputs comparable, divide the value from dictionary.tables by 1024.
By testing I found that there may be slight differences between the filesize value reported in dictionary.tables and proc contents (proc contents has the correct one). But that is in the range of a few K and therefore negligible.
There is also a slight difference between file size and the du value, that depends on technical aspects of the filesystem interna. Also negligible.
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