How to reduce Chromium OS image size?

I want to boot Chromium OS in qemu, but the size of chromiumos_image.bin is 6.4G is larger than the limitation of QEMU (4GB). Hence, I try to reduce the size of chromiumos_image.bin to

chromiumos_image.bin is built from by running 

./build_image –board=${BOARD} –noenable_rootfs_verification dev

and I need to run 

./build_packages –board=${BOARD}

first. Thus, I guess I should do something in these two steps to reduce the size of image (such as remove some package?). But I have no idea about this problem. Hope anyone can do me a favor.