At the time of this post, the size (on disk) of my Bookmarks was 3,088,384 bytes. The size (on disk) of my BookmarksExtras however, was a whooping 438,525,952 bytes.
I deleted few folders of bookmarks (each containing hundreds of links), reducing the size of Bookmarks file from 5,627,904 bytes to 3,088,384 bytes, but the size of BookmarksExtras didn't decrease at all.
So I then experimented a little bit, and here's what I found.
I placed most of my bookmarks in folders.
I usually deleted my bookmarks by right-clicking the folder and choosing the "Move to Trash". When I do that, the BookmarksExtras didn't decrease.
But when I tried selecting the individual links and choose the "Move the selected to Trash", the size of BookmarksExtras decreased.
I don't know why this happened and if this just happens to me, but what I need now is a method to "clean up" those obsolete thumbnail images stuck in my BookmarksExtras. Please help, anyone.