Busy sites that use a lot of forms can make a lot of entries in the cache_form table. This is particuarly true of sites using Ubercart, Commerce, or other modules like Fivestar, Ideal Comments, or Hierarchical Select.
This is a very common problem which I have seen cause problems for my clients, especially when database replication is involved. A search shows how widespread the issue is: https://www.google.com/search?q=cache_form%20big
$expiry = 21600; from a hardcoded variable to
variable_get('cache_form_expiry', 21600);, users can choose how often entries are pruned from cache. With shorter lifetimes, the form_cache table will be truncated more often, and will not grow as large.
I would suggest making the default expiry one hour, rather than 6, also.
None, subject to community review (especially of the comment).
User interface changes