"Line spacing" is determined by the design of the report: the height of the
report's sections, its controls, and the vertical space between them.
For example, Sections have a property "Can Shrink". Setting this to Yes can
also close a vertical gap, in some cases.
Another example, if the data in your Detail section appears double spaced,
make sure the controls in that section are only as tall as they need to be
for their font size and that they hug the top of the section. Then drag the
bottom of the section up so that it hugs the bottom of those controls.
HTH,