5MB doesn't seem like a lot of graphics. Most of the overhead is the crosswalk engine, and you're not going to be able to do anything about that.
FYI, C2 re-compresses your graphics on export by default. For maximum compression, select brute.
TiAm Yeah!. I just came to know about that crosswalk takes much from my friend recently, but still looking for some possible ways to reduce the size. Anyways thanks for the tip though, will use it.