FE does not expand the flash, it only copies the content of /usr to the USB device and mounts the device to /usr on bootup.
I'm not sure if FE deletes the content of /usr in flash memory after installation. But you can check it by yourself
df -m /usr
mkdir /tmp/rootfs
mount -o bind / /tmp/rootfs
du -sm /tmp/rootfs/usr
The last command shows the used space in MB of /usr of the flash memory.
If there are any data in /tmp/rootfs/usr , you can delete it - then you will have more free flash space. But by your own risk! The box will not boot anymore without the USB device.
As @Hilfsbereit already said: Flodder is the better solution to "expand" the flash