You have two choices. You can leave both views with a text size of auto, in which case it will automatically scale.
Or you can break the link for text size so that you can have 16 in your tablet view and something less than that (12, maybe) in your mobile view.
Here’s how to break the link. In groov Build, click your text area gadget in the workspace to highlight it. Then look in the Properties pane (upper right). See the small buttons at the right that have what looks like two links of a chain in them? Click the one next to Text Size, and then you’ll be able to set that property separately in the two views.