first of all a very good game. i play it alot when in office. Some improvement i can suggest are : -
1. Buttons are not hard to use and i don't think you even need to make them large. Just move the right button to the right side.
2. Ability to remember the high scores. Its always nice to see you top scores
3. Background (I guess you are already working on it)
I guess this is it for now but i have got tons of idea (only if i know how to program
).
If you keep on developing this game. i will keep on giving the ideas.
Keep up the good work.