int uncompress(char *dst, 
               int *dstlen, 
               char *src, 
               int srclen, 
               char *win);

This function resides in the startup library and is responsible for expanding a compressed OS image out to full size (this is invoked before main() gets called). If you know you're never going to be given a compressed image, you can replace this function with a stub version in your own code and thus make a smaller startup program.