Fix inconsistent vertical spacing?

I have a project with a piano + string trio. Using landscape and size 2 in page size.
Why do some pages have all the staves spaced super far apart and others super crammed? How do I fix this?

This sounds like the difference between both systems and the staves within them justifying, vs only systems justifying, but the staves within those systems only using their minimum gaps.

Try increasing the 2nd justification value in Layout Options > Vertical Spacing to something high, like 95%. You can see what % full each page is vertically by looking at the frame fullness indicators.