I'm not sure what all leads to the memory limit, I'd assume traffic accessing the webserver and php script utilization over time you'll have increased values; as no webserver/site/traffic is exact it'd really be impossible to have an end all be all for configurations, tuning your webserver according to your needs is why there are so many configuration variables.
The "core information" for memory limit is pretty lacking, but you can see the section on their manual
here, the page suggests though (just above where that link takes you) that the default they specify is something like 128mb.
