SQL Server 2016 memory optimised table: Precreated files constantly growing
Question title says SQL 2014, but question text says SQL 2016. This issue is not necessarily a bug in either version. 

Assuming db is in full recovery, in order for file merge to occur, transaction log backups and checkpoints must occur. If running simple, checkpoint alone will take care of file merge and clearing the log. Please see this blog post:

https://blogs.msdn.microsoft.com/psssql/2017/03/06/why-am-i-getting-so-many-checkpoint-files-when-i-have-in-memory-oltp-enabled/

But @Slowry is also correct, in that you must be fully patched. 

There is no knob you can turn to affect file precreation, although the required storage footprint is less in SQL 2016. There is no way to control the size of checkpoint file pairs, containers, or the memory optimized file group (in any version of SQL Server that currently supports In-Memory OLTP).