1708 Device usage calc fails on large long playlist

[chrisjj]

On MTP device, with MM device profile having no auto-conversion: http://i.imgur.com/KZE6Ahx.png :

1) On device profile auto-sync, ensure no files selected and verify To Sync: 0 B Audio: 0 B: http://i.imgur.com/mWVuB23.png
2) Checkmark a playlist containing e.g. 1215 files, 6.9GB http://i.imgur.com/djGVjNK.png
3) Inspect To Sync and Audio

Expected: To Sync and Audio each 6.9 GB
Observed: To Sync and Audio each 412.7 MB http://i.imgur.com/S7PVkeU.png

No such fail on a smaller & shorter playlist: http://i.imgur.com/AzezSA7.png