If you select the
TabContainer in the scene tree and look at the bounding box shown in the canvas area, you'll see that it contains the tabs themselves. If you select a child of the
TabContainer, you'll see the bounding box is only that child (below the tab set).
So, I'm not sure why you'd expect both the
TabContainer and it's children to be the same height. One has tabs and the other doesn't. Maybe I'm missing the question entirely...