So if you want the queries to return the same information you need to specify a schema owner for the dba_segments view.
select sum(bytes) from dba_segments where owner='THISUSER' AND segment_type='TABLE';
In general views that start with user only show objects in the schema that you are logged in as, while views that start either all_ or dba_ show objects in every schema. The difference between all_ and dba_ is that the dba_ views have columns the all_ views don't.
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