Your problem may be with the update mechanism of thumbnails, rather than with the cache size. The problem with Windows 7 is that when the folder is updated, it will insist on deleting and recreating the thumbnails for all the files in the folder.
There are two possible solutions to the problem.
First solution is to turn off thumbnails creation for that folder, so Windows will list the videos as simple files. The negative aspect here is that the videos will need to identified by file-name and/or date.
This approach is described in detail in the article
Speed Up Windows 7 Folder Loading Time
and involves : Go to Properties of the folder, Customize tab, and under
Optimize this folder for set the content-type of the folder from the drop-down menu to Document, and hit OK.
Second solution involves monkeying with the folder-permission, in effect turning-off the permission to delete and recreate the thumbnail cache every time. This is described in the article
Stop/Prevent Windows 7 From Deleting Thumbnail Cache.
I remark that I have no idea how well this will work or what other complications it will cause,
so I can take no responsibility for it.