The "leading" (official term for the space between the lines) is determined by how tall you make the object. Make a test by adjusting the height of the object a little taller .. or a little shorter .. and you'll notice the spacing between lines changes.

For example, if you have the field defined tall enough for 4 lines, notice how much "extra" space is defined after the last line of the field. Fox takes that extra space and divides it evenly between the lines of the field. So if there's a lot of space after the last underline in the field .. the lines will be spaced farther apart. If there's very little space after the last underline in the field, the lines will be closer together.