Is that 3gb per video or an overall limit? I think it is one of the mysteries of Lightroom.
Purging manually is the only option I know. You could perhaps make a habit of purging before importing new videos to work on.
I tend not to import videos into LR as I view them in Bridge and have Premiere for editing.
You'll see a more complete version of this answer under the first thread that comes up in the search 99jon posted.
The problem is that the video cache limit is only for LR caching videos. The other contents you're seeing comes from LR initiating a call to the DynamicLinkMediaServer whenever you click on a folder during the Import process. The DLMS scans ALL FOLDERS UNDERNEATH whatever folders you click in when you import and every video it finds is cached in the video cache and LR's limit doesn't apply since the DLMS is a distinct program and really knows nothing of LR. You can avoid this problem by renaming the DLMS executable to some other name.
I wasn't really aware about what Bridge was. In fact, it is exactly what I wanted to organize my files: something light, customizable and fast.
Thanks for your message, I discovered it and adopted it instantly.