Why are these two owrt-yun 1.5.3 bin images on your Website different size?

Hi there,

Why these difference in sizes??

$ ls -lh openwrt-ar71xx-generic-yun-16M-squashfs-sysupgrade.bin ... 15M 14 Nov 2014 openwrt-ar71xx-generic-yun-16M-squashfs-sysupgrade.bin

$ ls -lh YunImage_v1.5.3/* ... 54K 13 Nov 2014 YunImage_v1.5.3/openwrt-ar71xx-generic-linino-u-boot.bin ... 1.2M 14 Nov 2014 YunImage_v1.5.3/openwrt-ar71xx-generic-yun-16M-kernel.bin ... 7.5M 14 Nov 2014 YunImage_v1.5.3/openwrt-ar71xx-generic-yun-16M-rootfs-squashfs.bin

Not sure what the sysupgrade includes but even if kernel and rootfs they don't add up to 15MB unless the sysupgrade on is padded for some reason?

Ane the base images from here 16MB:

http://arduino.cc/download_handler.php?f=/openwrtyun/1/YunImage_v1.5.3.zip

The sysupgrade image is from here 7.9 MB:

https://arduino.cc/download_handler.php?f=/openwrtyun/1/YunSysupgradeImage_v1.5.3.zip

TIA for any explanation.