Mixed Staff Spacing

[Gilles]

OK, now the staves have the right sizes (see message with subject 
"Mixed Staff Sizes").  But reducing the size of a staff did not
reduce its vertical extent: The system (combined parts staves) takes
the same vertical space as before.
How can I reduce the space between the "Flute" staff and the "Piano"
right-hand staff?

Thanks,
Gilles