It is the same reason Dropbox doesn't include anything in the web interface to find large files.
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Edit: tossing in some options to accomplish this; not really related to the current discussion:
Space usage analyser for Dropbox? | https://webapps.stackexchange.com/questions/47440/space-usag...
Unclouded - Cloud Manager | https://play.google.com/store/apps/details?id=com.cgollner.u...