Adam,
That's correct -- currently there is no user interface (like VIEWTABLE) to allow you to subset the data in place. All of the filtering steps in EG are recorded as steps in the project so that you can re-run them as a flow, and each step must result in another output table.
To save space, you can alter your query/filter step to create a VIEW instead of a TABLE. Also, if you want to really modify the data permanently in place, you can create a code step that performs a DATA step to do that work:
[pre]
data work.class;
set work.class(where=(age>14));
run;
[/pre]
We are looking at ways to make this better in a future relase so that you can apply adhoc filters without creating new tables.
Chris