1. Try using the same font size throughout as much as possible. Having different font sizes on a single page make it a bit more difficult to read because your eyes have to keep adjusting all the time. 2. Use larger font sizes only when certain words or phrases are particularly emphasized. 3. You don't need to fill up the entire word bubble with text. Leave a generous amount of white space around the borders to make it look less cluttered.
On the plus side, your use of different colors on Page 17 to show which girl was speaking -- especially on the "joint speech" bubble -- was a clever touch.