Great job with minimalistic art and level design!
I felt that early levels were a little bit too repetitive, but some of the puzzles later on were great. I especially liked the puzzles with two exit points: one for big guy, another for little guy. But I constantly got confused which one is 'I' and which one is 'II'. Maybe you could have made small icons based on their appearance instead? Color-coding the buttons and doors would help, too.
And the last on my wishlist would be the ability to skip one or two levels that you're stuck on. I got stuck somewhere in the yellow area (the level on the screenshot). I would try out some later levels if I had the ability to skip that one.
Overall, a very enjoyable experience!