Are you saying you have three notebooks with 300 sections? I agree that shouldn't be causing a memory leak but you realize that each section is saved as a ONE file, right? I don't know if you need to access them all at once or not, but it might make sense in terms of memory usage to archive some of the notes you are not using.
As for your problem at hand, does memory spike when you text as well or only when inking? If it's only while inking, open OneNote and don't immediately try to write something. Open a new section or page and type something, then go back to the page you want to work in. See if this works as temporary workaround for the memory leak.