Apparently that's something that started with rev56, HANA preallocates 30GB of RAM even if not using all of it. It was explained by Juergen Schmerderhere: Developer Center HOWTO - Update your SAP HANA, developer edition on AWS
SAP HANA seems to consume more and more resources with every revision. With Revision 56, a HANA Server pre-allocates 30GB when idle. So you might think that the minimums RAM to throw at the server would be >30GB, which rules out the AWS m2.xlarge instance type that only comes with 17.1 GB. However, if you look on the OS level (e.g. with top, free or meminfo), you will see that the amount of physical memory consumed by a humming HANA server is indeed much lower. The reason is that this pre-allocation is logical - it doesn't really allocate the memory, but every HANA service "reserves" the right to grab a high amount of memory. Details can be found in chapter 9.10 of the HANA Administration Guide.
Best,
Henrique.