nice game! I played for 5 or 10 minutes before i started getting bored. i think the increase in difficulty comes too slowly, theres no real tension or feeling that failure is imminent if you make a bad move.
perhaps the playing area is a little too large? giving too much freedom. one way to make it more challenging could be to introduce the larger pieces sooner, and as time goes by, decrease the occurrence of small triangles, as they become the vital part to fill in odd spaces. is it possible to make 4x4 squares? i couldnt seem to do it.
also, the positioning of the pieces didnt feel right on my galaxy s4. they seemed to be too far from where i touch my finger, which you might have done to make it easier to see, but it felt strange.