Large Cache a Problem? What to do?

My Cache seems to be unreasonably large. It shows 15.5MB, even though all other tables together are much less than that.
How can the cache grow larger than all other tables combined?
Should I be worried? I run cron about every 8 hours and I do not really want to flush the cache to keep my database requests low, but I am wondering why it keeps growing.
I optimized the database via phpMyadmin but it didn't make a difference.
The cache also shows a lot of records:
Here is an example:

table records size:

cache 2973 15.5MB
files 336 28kB
menu 122 7.8kB
node 161 297.8kB
url alias 351 38.6kB

I have a lot of cache entries that look like this:

cid: filter:3:cc802c3671ad58eddddf3c42969798c7

I guess those are normal ones:
cid: http://www.someurl.com

Wonder what the filter thing is.

Anyone cares to elaborate?

Thanks

Andre