The same thing happens if the browser window is narrowed considerably.
These two fixed heights are preventing their containers from extending with the contents:
engST.css (line 62)
engST.css (line 67)
I suggest that you delete them.
You can put a little space beneath the button by adding:
padding-bottom:20px; /* as desired */
The vertical margins on some of these objects are defined in percent. Not a good choice. Although it's a vertical margin, a percent value is interpreted as a percent of the width, not the height. I recommend changing them to pixel values.