In Opera, it limits you a only a 400MB disk cache. I test a lot of graphically enhanced sites and files that easily fill that up by the end of the day. At least once, maybe twice, per day I need to clear the cache and close the browser down and restart. Is there way to expand the disk cache beyond 400MB by mods or hacks?
It looks like they limit it so as not to overly effect the performance of the machine and/or browser.
From the Opera Knowledge Base:
Please note that a larger cache size might actually slow down older systems. In general, we recommend that users with older systems (early Pentiums and older) set their disk cache to 2 MB and memory cache to automatic or 4 MB.
A very large cache might also cause Opera to take longer to start, if the cache is filled up. You can avoid this by enabling "Empty on exit".
If you enable automatic memory cache, it will take up about 10% of the available RAM on your system.