File size issues

I am having an issues with file sizes on my WD HD TV box.  When I connect my external HD that has a bunch of movies on it, it is not reading the correct file size?  I have some files that are 8+ gig and some that are 4+gig but when you look at the file size on the WD TV box it says anywhere from 750mb to 3.9gig.  And I have had a couple movies where the movie stopped playing half way through the movie.  But when you plug the drive into a PC is sees the correct size and will play the entire movie.

Does anyone know what the problem is?  I do have the latest firmware update on the device.

Please help because this device is useless if it cannot read and play the movies I have on the drives.

What format is the drive?   Fat32 or NTFS?