Sap hana sizing is done as part of sap hana first installation requirements.
Sizing calculation takes into account source database size before migration, data growth estimation over few years and of course other factors which won't be detailed here.
Today, you realized, free memory availability has reached a critical point. It could be either because of unexpected data load or because of unexpected new databases added to the sap hana system.
Whatever the reason, you are confronted to find solutions. First of all what are the different possibilities to free memory space. Check with the application users and
analyze used memory content which could be purged
But after best effort, You have come to the conclusion that you have to migrate all your databases to a new environment or you have to add more physical memory to the existing system.
Now let's say, you are lucky enough to realize that your sap hana system run on one server which can accommodate more memory bars. The migration headache can be avoided.