The mystery of vertical spacing

[Paul Hodges]

One of the hardest things to find information on, it seems to me, is how 
to adjust vertical spacing.  There's lots of information about adjusting 
spacing within systems, but little about fitting systems on a page.

In the example below, there is lots of space between the systems.  The 
global font size is 20.2; if I increase the font size to 20.3 Lilypond 
will no longer tolerate having these systems on the same page.  Why? - 
there's plenty of room.  More to the point, how do I tell LilyPond to 
allow the systems to get closer?

Paul