Running out of space for Docker seems to be a common problem and there doesn't seem to be a single good answer on how to solve that. I created a VirtualBox CentOS 7 VM via Vagrant. This is a 40GB disk image. I then increased the disk size to 100GB via VirtualBox tools. Even with 100GB, I get a similar error when I try to import the sandbox. I haven't yet tried to use the -g option.
The problem is that Docker still uses a virtual machine behind the scenes to run containers. The default storage size for that VM appears to be 20GB. I've found a number of threads from people wanting to see that increased to 100 or 200GB, which seems reasonable to me. Unfortunately, I don't think that has been changed/released.
Having said all of that, Amazon Linux uses upstart at the init system. Have you tried making your changes to /etc/default/docker instead of /etc/sysconfig/docker?