I agree the pagecache size here seems far too low, I think we may need to establish a minimum to recommend even in the cases of a small db. It's highly unlikely it would stay that small. That said, you only have 3.5 gb of memory or so on that system, so you don't have much to spare. Unless you are only going to be working with small graphs, you may want to consider upgrading to an instance with more memory.
Pagecache sizes do not have to be a factor of the heap size, they are mostly unrelated. While the heap is basically a scratch area in memory for query execution, the pagecache is more correlated with the db size. Having as much of the graph in the pagecache is a means to minimize disk I/O. With insufficient pagecache coverage of the database, you won't encounter memory errors, but may end up with inefficient querying due to pagecache misses and needing to go to disk.