Most likely DVD-A thinks they are non-compliant, and needs to re-encode them, and it is assuming a higher bitrate than what was used in your orginal encoding (DVD-A will base it's calculations on 8 Mps, unless you change the default).
When you do Optimize DVD, does it show it needing to re-encode?
How did you create the MPG files? Are you sure they are DVD compliant?