There are two things to review when your report is showing too much white space.
1. If you have a text block in the page footer, it will only print at the bottom of the page. This will cause white space between that text box and the detail section (because the page footer places itself at the bottom of the page based on the page margins.)
2. Another things to do is do "fit the section". This will make the section the appropriate size the data included. To do this right click within the grey portion on the left hand side in the section in question and choose fit section.