SQL SERVER 2016 In-Memory Database
Database size can exceed the RAM capacity as it could contain normal disk based tables as well as In-Memory tables. However, the size of memory based tables should not exceed the available memory. This available memory could be less than the RAM capacity as there are other processes that will be consuming some amount of RAM.

List of various out of memory issues that could arise: 


Resolve Out Of Memory issues


The memory required for In-Memory tables is not just the table size but it should also include the following:


indexes 
row versioning 
table variables 
growth


Estimating memory requirements for memory optimized tables: 


Estimate Memory Requirements for Memory-Optimized Tables