Fortigate Memory Size

 maybe this link will help to answer your questions, https://yurisk.info/2021/03/14/Fortigate-Firewalls-Hardware-CPU-model-and-number-Memory-size-datasheet-table/ 

FGT-60F  2GB

FGT-50G  2GB

FGT-70G  4GB

My personnal experience : we decommissioned about 50 60F and replaced them with 70G, no more RAM or CPU issues they work flawlessly, we are considering 50G for very small site only