There were some changes to the MathObjects library to address memory leak issues about 5 months ago. These are not yet included in an official release, so you would not have gotten them if you updated to 2.4.0. Perhaps you should try updating your pg directory to HEAD rather than 2.4 and see if that improves your memory situation. You can do that without updating webwork2, as the two directories are pretty independent.
WeBWorK Main Forum
Memory Usage & Problem Connecting with mySQL server
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