In the design, make the text box only big enough to hold the <Exp> placeholder text. Text boxes have a "can grow" property turned on (true) by default, so if more room is needed to add lines of text, it will expand vertically to hold the text. Unless you specifically turn "can grow" off (false), it will do this. On the other hand, even setting "CanShrink" to "True" does not always have the desired effect.
So start with the box at the minimum size in the design, and let it grow dynamically to hold the text, rather than make the box big in the design and then try to make it shrink in the output.