SQL Server In-Memory Table Conversion
Microsoft recommends 2x data/index size as a starting point for memory allocation, but that's a generalization. 

Is any of the data and/or indexes in the source table compressed? 

How much memory on the server? 

How much memory allocated to SQL Server? 

When you created the resource group, did you bind the database to it, and then offline/online the database to make the binding effective? What percent did you allocate to the resource group? 

Please post the result of: 
SELECT committed_target_kb
FROM sys.dm_os_sys_info

And: 
SELECT @@version

Agree with @sp_blitzErik that an atomic insert of 100 million rows to a memory-optimized table is not the best way to migrate data, but not necessarily because it will require less memory - batching will perform much better.