Impress; why can't font size be increased to 16?

cf. Image. I have a slide with 2 text boxes. I’ve copied the top one.
In the bottem text bos, for some unknown reason, after I hit enter to make a new line, the text downsizded from 16 to 10, and the text wont be resized to 16. Anybody has a clue how / why this happens?

Wait a minute, I continued working on the same document, at the top text-box adding text and automatically it reduced the size to fit the text in the box! That must be the feature. But I can’t find where that is hidden. Thy