You assumption is wrong, the row height did not stay the same. The row height is the distance between the top and the bottom of the row, in other words the occupied space between the rows above and beneath it. Your sketch clearly shows a raised distance, and that’s what really happens.
The problem with your expectation is a completely different: you expect the handle to the left of the cell to move upwards staying at the center of the text but the handle remains at the center of the entire cell, including the empty border you have added.
As of Java 7 and BasicLookAndFeel and its descendants (like Windows and Metal) the vertical centering of the handle line is hardcoded in the implementation.
So if you want the handle to stay at the center of the text the only practicable solution is to keep the text centered by adding the empty space equally to the top and the bottom.