In complex files/production context, it can be very useful to be able to quickly see the size used by data-blocks, which ones are very heavy, etc.
This could be done e.g. by checking the last undo step, size is stored in memory chunks, so by doing a quick loop over all BHeads we should be able to get the size of each ID on disk? Or maybe even more efficient, storing that info in IDs after each undo step is written, generating it would then be almost for free.
Outliner could then have a optional column to display that info.