Required disc space

I wonder how DVD-SP counts the used discspace:

I have one 155 min long video.m2v which is compressed to size of 4.25 GB it´s audio.AC3 is compressed to 160 MB and the menu´s still-image background has a 3.5 min long audio.AC3 track 4.6 MB which should be all together less that 4.5 GB, but DVD-SP declares in disc meter that 5.1 GB is used.

Importing only the main movie 4.25 GB brings the disc meter to 4.7 GB and bringing in the 1.6 GB audio track raises it to 4.9 GB...