We converted a database table to be in-memory and the corresponding memory-optimized filegroup takes 1GB on the disk
I would like to close the loop on my own question.  We worked with Microsoft support on this and the bottom line is : "it's by design".  I strongly believe that this is a flawed design and should be changed.  The Microsoft support engineer opened a feature change request for this.  In addition I created a "MS Connect" item about this.  Please up-vote it if you agree with the request: 
https://connect.microsoft.com/SQLServer/feedback/details/3116296/enabling-in-memory-feature-for-databases-takes-up-huge-amount-of-disk-space-for-checkpoint-file-pairs-cfps-in-cases-when-there-are-many-databases-on-a-sql-server-instance