There are situations in which the error_log associated with a domain fills up a good part of the disk space consumed.
Lets find what are they logging ! Is it something like,
[Date America/New_York] PHP Strict Standards: Non-static method JDispatcher::getInstance() should not be called statically in /path-to/file.php
We can see that PHP Strict-Standards errors are being reported here. As each and every strict standard errors is being reported, error_log will consume huge amount of space.
This is a change which has been seen in the newer version of PHP, which now reports E_STRICT errors on default.
To get around this issue, disable error reporting for strict standards, by adding the below line to PHP configuration file.
error_reporting = E_ALL & ~E_NOTICE & ~E_STRICT