Unlike 3ds Max, which uses inches as its base system unit, MotionBuilder and Maya both use centimeters. To compensate for this, 3ds Max users are generally instructed to switch their Display units to centimeters, and leave the default system units setting (inches) unchanged. This creates problems when exchanging files between other software applications.

For example, if you export a 3ds Max object when you have your Display units set to one measurement (such as centimeters), and your System Units set to another measurement, the FBX exporter only considers the 3ds Max System Units, even if they differ from the Display units.

This becomes more complex if you then save the FBX file in MotionBuilder and import it back into 3ds Max. The FBX Importer evaluates the 3ds Max System Units, but once you save the FBX file in MotionBuilder, it is converted to centimeters. The FBX Importer accordingly converts the incoming FBX file to match the 3ds Max System Units. This now scales your object even though you had no expectation of scaling because your Display units have always been in centimeters.

If you want to exchange FBX files between 3ds Max and MotionBuilder, change your System Unit settings to centimeters to match. If you want to exchange FBX files between 3ds Max and Maya, change your 3ds Max System Unit settings to match the Maya Working units.