I would recommend experimenting with Layout Options Vertical Spacing and Vertical Justification settings. Pages 4 and 6 are over 80% full, so in accordance with the default justification settings, they are justifying only the distance between systems, and not the distances between staves. You can change this to a higher value like 90%, and you might like the results.
I often find that for scores of smaller ensembles like this, I like the results if I set the threshold for vertical justification of systems only to 100% (essentially so that the distance between staves will always be justified), and then experiment with a larger value for my inter-system gap (in ideal gaps) like 14-18 spaces.