Creating less/more space between two systems

In general, to add space between two random systems, see http://musescore.org/en/handbook/break-or-spacer.

However, the specific case you are showing is different. What you are seeing is that the first page is exceeding the page fill threshold and is thus triggering the automatic stretching out of the systems to fill the page. Thus, you are seeing more space between the systems on that page than you actually have set. So if you do increase the space between systems globally (style->edit general style), it won't actually affect that first page, at least not unless you increase it so much that it can't even fit three systems on the page any more.

So you first step should be to increase the space between systems so that pages like the second that do not exceed the fill threshold don't have smaller space than you actually would want. Once you have a decent amount of space showing on the second page, you might still find the systems closer together than on the first page. So the way to get them more even is to add a spacer below the last system of the second page, then stretch it down to the bottom of the page to trigger the page fill on that page too - just play with the amount of spacer to add until the effect looks pleasing.