Probably what it's doing is using an expansion/compression feature and converting the audio to LPCM. If it does that, 90 minutes of video is too long for a single disk.

Look around for a check box somewhere. There should be one that basically says burn "as is" with no disk compression.

First time I tried Toast I had a 1.7gb file that got doubled in size when I burned it. After being puzzled for a while I found a setting somewhere and unchecked it. Can't remember where it was though, and IIRC it was Toast 8.

A single layer DVD will handle 4.7gb total (4,483mb to be exact). A dual layer actually only handles 8.5gb.