On Access, the best I've been able to do is shorten the height of the sub-report and set the 'can grow' property to "yes". It still take a few millimeters of space but is better than otherwise using up the entire height of the fields.

You might consider making the 'final product' in Crystal Reports. Crystal has a very strong feature in that it can overlay a section of a report -underneath- another section. I've also been able to tweak sub-reports to take a minimal amount of space.

I've had fun with sub-reports in both Access and Crystal and If it's important enough make the sub-reports not expand then you'd be better served by Crystal.