i tried 18.06.1 on huawei hg255d,after compiled, the openwrt-ramips-rt305x-hg255d-squashfs-sysupgrade.bin is so small that the size is only 3Mb, and can't login via web interface, than i login with ssh,and found "opkg list" shows only a few packages in the system . though in the bin/packages/mipsel_24kc/packages ,all the ipks are in there. so i can't understand why the build process not pack the selected packages in the image?
Stable builds come with a web interface, so either your install is kinda broken or you flashed an unstable or custom build.
Even though /root may only be 3 MB, you still need to allocate space for the kernel, and for the overlay (which stores settings and additional packages).
it turns out the download didn't complete.