Keynote fonts in text box change size when entering data

When I enter text into a text box using the font saved in the master slide, everything works fine until the text box reaches a certain point of being full. Then the font starts shrinking even though the text box is not full . The font size in the inspector does not change even though the font itself does change.

So now the font size does not correspond to the size indicated in the inspector. How can I keep the font size from changing when the size of the text box changes?