WebPlayer image caching and compression

When using web cache in WebPlayer, cache size is too large because of the image asset bundles.
AFAIK
1. WebPlayer can use cache only by WWW.LoadFromCacheOrDownload()
2. WWW.LoadFromCacheOrDownload() can use asset bundle only.
3. Downloaded asset bundles are uncompressed at local storage.

Is it right?
Then how can I minimize the image asset size?