There will be a system generated file contains info about how much space occupied by each dataset and many information
Note :SYSLOC.SMF.SMFDSRPT this GDG contains info about who edited which JOB.ie)which job created,used,edited dataset and all.
In My project it starts with B5717.
Please check it.
Just run a DCOLLECT and process the resulting dataset,
a little parsing will have to be done to split the datasets names on the high level qualifiers..
But then this should be a task that any storage support has done zillions of times
and the reports about space occupancy should readily be available to You
but in any case You might find useful info on how to process dcollect data using dfsort
starting from this link