Switching back and forth the full screen mode on a desktop whenever the user clicks the Start / Stop button is too much in my opinion. Perhaps adding a "Full screen" button in the side bar would be beneficial. I'll probably add that for the next release.
On mobiles the situation is different. The available screen real estate is so small it's much better to maximize the screen whenever possible.
I didn't understand if the caption is too large or too small in your opinion. By the way this hasn't been changed in v2.0
You can make the title/label and comment any size you like. Under Projector > Advanced > Custom code > CSS, you can enter either of these lines, or both. The first controls the title/label size and the second controls the comment size. Adjust the values to get what you want (small increments, like 0.95 for larger, and 0.75 for smaller):