Keeping the mobile game size minimum as possible

I have done a game for mobile using Unity 2D . When i create an apk file its around 10 MB
and when i generate an output for windows phone 8 its xap file is 4 MB .

I have few doubts and need suggesstion so that i can keep the game size to its minimum
without the loss of quality.

All my original assets (background,player,enemies etc) are in .psd photoshop format
which have bigger size than corresponding png files , but i dont think that should make any difference in game output size since unity internally converts the textures while compiling the game output.(???)

I would like my game to look awesome whether its being played on a tablet or a mobile

Read these articles from Unity documentation:

  1. Optimizations

  2. Practical Guide to Optimization for Mobiles

And you can also find some great tips on Unity Forum posts and search on Google to find some great tips.