When a text-input is presented as MultiLine and has a specific height, this height definition is added with !important in the additional custom css at the end of the page for field #id
This addition of !important prevents end-users from expanding the input, which is a standard feature in browsers, but gets blocked by this height !important thing.
This also applies to width - if that is set.
Support ticket: 7463
Below, the multiline input is set with a 150px height. You cannot change the height (dragging vertically, but you can make it smaller (width, dragging horizontally).