InfoPath forms are all about being dynamic, so it's important that the sections in your form grow and shrink appropriately. For example, if someone deletes an optional section from inside another section, the outer section should shrink accordingly.
Of course, sections will grow/shrink by default after you insert them, but InfoPath will change it to a fixed size if you resize the section, since we don't want to disobey your command to be a specific size. So in the example above, the outer section will stay the same size even when the Notes section isn't there -- it will just have a bunch of white space in its place.
The trick: Use "auto"
If you resized a section by accident and want to return it to the default, just open its properties, switch to the "Size" tab, and type auto for the height. Now it will only be as big as it needs to be and never bigger.