Mongo DB crashed (Free tier) and I couldn't restart it until I increased the Volume Size to 50GB.

That's super common, Mongo has a very hard time operating with a 500MB volume.

How is it possible that systemMemSizeMB is 257GB? Is there any way to affect it?

That's the host machines memory, your Pro plan allows Mongo to use 32GB of that.

Thank you for your response. I don't understand why I couldn't restart it. Could it be that the virtual memory of MongoDB grew too large and caused the issue? Hopefully, the problem won't recur with a larger volume size. However, there's still no certainty about what actually caused that error.