fun concept! I think it has a lot of potential.
There's a bug with the big guys attack not always breaking the boxes.
If you set your stretch mode to canvas items, you won't have to worry about your game losing screen space in small windows or full screen. I couldn't see the start button until I full screened.
The small guy should keep a little bit of momentum when you jump, just to help feel like you're going fast.
The blind jump in the last level is a bit mean.
I really liked this entry overall though. With some polish and a story, this could be a really cute game!
I forgot to mention, you did a fantastic job with all the art!