This doesn't make a lot of sense to me if they are BAM files, perhaps if you made a screenshot it would help. By default each BAM file is displayed in a separate panel with a scrollbar. The track height of a BAM file is the height within that panel, it will not have any effect on the height or screen area of the other BAM files, which are in other panels.
To get more control of the track heights for your situation you can select "View > Preferences > General tab" and select "Display all tracks in a single panel" as shown in the screenshot. Then quit IGV and restart and load your BAM files.
Additionally, you might want to not show the coverage track with that many BAM files. To set this up by default select "View > Preferences > Alignments" and un-check Coverage Track in Track Display Options. This will give you more room for the alignments.
It is difficult to see 6 experiments at the level of alignment data in one screen, Variants (VCF) files are usually more appropriate for this type of comparison.
