After you do a build, open up your editor.log from within Unity by going to Help --> Open Editor console log.

Scroll up from the bottom a little and you should see a section that looks like this:

Textures 10.3 mb 79.1%
Meshes 7.2 kb 0.1%
Animations 3.8 kb 0.0%
Sounds 62.3 kb 0.5%
Shaders 0.9 kb 0.0%
Other Assets 30.9 kb 0.2%
Levels 64.9 kb 0.5%
Scripts 62.3 kb 0.5%
Included DLLs 2.5 mb 19.0%
File headers 23.6 kb 0.2%
Complete size 13.0 mb 100.0%

Used Assets, sorted by uncompressed size:
...
2.5 kb 0.0% Assets/Textures/Ui/hud-lifeman.png
2.3 kb 0.0% Assets/Animations/spike-typeA.mb
2.0 kb 0.0% Assets/Animations/pow-star.mb
...