Auto-deleting the cache would only solve one problem you’re having (disk space), but there are 2 other problems -which I consider more important- that auto-cleaning can never solve: 1. you will be generating new autoptimized JS very regularly, which slows your site down for users who happen to be the unlucky ones requesting that page 2. a visitor going from page X to page Y will very likely have to request a different autoptimized JS file for page Y instead of using the one from page X from cache, again slowing your site down
So I actually consider the cache-size warning like a canary in the coal mines; if the canary dies, you know there’s a bigger problem.
You don’t (or shouldn’t) really want me to take away the canary! :)
Update 2016: since AO 2.0, inline JS is not aggregated by default any more, which should prevent cache-size problems from occurring. Easiest solution for cache size issues is to make sure “aggregate inline JS” (and CSS) option is disabled. Below HowTo remains relevant in case you decide to enable the aggregation of inline code.
Confession time: Autoptimize does not have its proper cache purging mechanism. There are some good reasons for that (see below) but in most cases this is not something to worry about.
Open two similar pages (posts).
Copy the source of each to a seperate file and replace all semi-colons (“;”) with semi-colon+linefeed (“;\n”) in both files.
Execute an automatic comparison between the two using e.g. diff (or “compare” in Notepad++), this should give you one or more lines that will probably be almost the same, but not exactly (e.g. with a different nonce or a postid in them).
Go to the autoptimize settings page and make sure the advanced settings are shown.
Now add the strings or filenames from (6) to “Exclude scripts from Autoptimize:” (which is a comma-seperated list).
Re-enable JS optimization.
Save settings & clear cache.
This does require some digging, but the advantages are clear; a (much) smaller cache-size on disk and better performance for your visitors. Everyone will be so happy, people will want to hug you and there will be much rejoicing, generally.
So why doesn’t Autoptimize have automatic cache pruning? Well, the problem is a page caching layer (which could be a browser, a caching reverse proxy or a wordpress page caching plugin) contains pages that refer to the aggregated JS/CSS-files. If those optimized files were to be automatically removed while the page would remain in the page caching layer, people would get the cached page without any JS- or CSS-files being available. And as I don’t want Autoptimize to break your pages, I didn’t include a automatic cache purging mechanism. But if you have a bright idea of how this problem could be tackled, I’d be happy to reconsider, off course!