Yeah, the line height used to happen automatically according to the text size. But in recent new versions (for some reason which I can't fathom) now you have to set it every time you change the text size.
Look on the Text tool control bar (not the Text and Font dialog). The field is just to the right of the icon which is 2 capital "A"s on top of each other (one blue, one black).
If you're using version 0.92.0, this option might be broken. Suggest either upgrading to 0.92.2, or go back to 0.91.