The line height is the line that an element with a font-size will sit in. The line height is made up with half leading that is placed on top and bottom of the font size you define to allow for descenders and ascenders.
Therefore text with a 30px font size will need to sit in a line of about 36px to display properly and depending on the line-height that has been set for the line.
You could set the line-height to the same as the font-size:
That will clip the size to 30px only resulting in the descenders being clipped as they now stick out of the line-height.
As mentioned above you should avoid using the height on the input unless you have no choice in the matter. If you want a height of about 30px then reduce the font size down a bit and set the line-height to about 1.2 or use a normal font-size and apply some padding top and bottom to create the extra height. This will allow the element to expand as required.