Some of the timestamps in the startup module are user-specific, which means a lot of different versions of the startup module are served. These all get run through the minifier, which puts them in the minifier cache. This caches the result by MD5 hash, and the cache entry never expires. This is wasteful but OK in a system like memcached that does LRU eviction, but it's much less OK in a system without evictions, such as the objectcache table.
According to the report at https://www.mediawiki.org/wiki/Talk:ResourceLoader#Cache_consumption_20440 this leads to something like 30 MB/day of cache growth, which is bad. We should redesign the minifier cache to behave more gracefully.
See also: