I have the same issue in my owncloud 10.0.2.
It seems to be the issue of server side that return with the wrong value of file size.
I used console.log to print out the fileData.size in 'apps/files/jsfilelist.js', and those 'Pending' folder size are negative.
After checking oc_filecache table, the 'Pending' folders in the oc_filecache has the size value bigger than 2G.
Also, when I add new sync folder into my owncloud client (Mac OSX version 2.3.2, build 4250), I saw the 'Pending' size folders in the Web UI do not have file size in the tree view of folder list.