Probably this is because of mmapped io access mode, which is enabled by default
in 64-bit VMs - RAM is occupied by data files.
If you have such a tight memory reqs, you can turn on standard access mode in
storage-conf.xml, but dont expect it to work fast then:
<!--
~ Access mode. mmapped i/o is substantially faster, but only practical on
~ a 64bit machine (which notably does not include EC2 "small" instances)
~ or relatively small datasets. "auto", the safe choice, will enable
~ mmapping on a 64bit JVM. Other values are "mmap", "mmap_index_only"
~ (which may allow you to get part of the benefits of mmap on a 32bit
~ machine by mmapping only index files) and "standard".
~ (The buffer size settings that follow only apply to standard,
~ non-mmapped i/o.)
-->
<DiskAccessMode>standard</DiskAccessMode>