The first approach is to adjust the shape of the balloon to better fit the words. Stretching this balloon out horizontally will definitely help the words to fit. If you have room in the panel try resizing the balloon to different balloon shapes to best fit the words.
The next approach is to adjust the line breaks in the words to force the text to make an oval shaped outline. For example a line break after "THIS IS" will make sure that not too much text is at the top of the balloon. Continue adding line breaks until the text makes a pleasing oval or circular outline.
The final approach is to change the words. If you can't get the existing words to fit in a pleasing way try choosing different words. Balloons benefit from small words. Long words make it difficult to get the shape you need - unless, of course, that word just happens to land at the widest point of the balloon.
After applying these techniques this is what a well laid out balloon would look like: