The linux kernel on your Ubuntu installation ship with a setting* which limits RAM extraction to has 1 MB.
If you do not wish that you can either recompile the kernel without it (obvious caveeat: you are lowering security!) or you can download and insmod the forensic kernel module fmem to work around this. That provides a /dev/fmem device without any security.
As mentioned by Opello: you can also use strict-devmem=0
in the kernel cmdline.
*: STRICT_DEVMEM=y
(see e.g. /boot/config-KERNELVERSION)