In principle, ArangoDB behaves similarly to MongoDB here. Both are essentially "mostly-in-memory" databases in the sense that they hold the data in memory and persist it at the same time to disk via memory mapped files. This approach is good for performance and if you run out of RAM you ought to shard your data.

However, MongoDB often uses a lot of memory for the actual data, since its BSON binary format stores the names of the attributes with every single document. ArangoDB detects similar shapes of documents (see https://www.arangodb.com/faq#how-do-shapes-work-in-arangodb) and thus avoids this particular problem.