RootsMagic is unable to import backups larger than 4 GB, at this time. The Family Tree Maker backup (.ftmb) file contains the database file and your media files, making some files to large to import. An additional step is needed in these special circumstances. It requires the .ftmb file to be unzipped in order to access the database file inside it. Once the database file is extracted it will direct import without any issue.
Steps to extract:
Right click on the .ftmb file and select "Rename".
Change the file name extension from .ftmb to .zip and hit Enter. Confirm Yes, you want to change the file name extension.
Next, right click on the .zip file and select "Extract All" to unzip the file. Confirm your folder location and click "Extract".
A new folder will be created with that file name. Inside you will find your Family Tree Maker database that can now be imported directly into RootsMagic using File > Import > Family Tree Maker.