The second partition of the eMMC (dreambox-data) is not mountet.
RAM is reserved for grafiks etc.
Why would you use more then 1GB flash for your image? The rest can be mounted and used for whatever you like.
Even with all currently available plugins and picons installed, you would never reach 1GB.
Picons are available for dm7080. There was nothing to change. But even with picons and plugins you might not be able to use all the available memory. Still, you can mount it. 
You can try DarkShadow Plugin, it allows you to use the 2.7GB recovery partition also for a copy of the root filesystem which is even bootable by the Plugin.
But it is a performance question, every extra inode produces overhead when the root filesystem is mounted, and tests have shown that 1GB ist more or less the optimal size where you get enough inodes even to hold thousands of picons and 2 images (usefull for the way dBackup flashes) and give minimal mount time and sufficient free space.
DMM simply uses the rest as recovery partition, like lots of PCs have hidden partitions for operating sstem recovery or backup.
The rest of the memory is given by the kernel to the Video chip, all these nice transcodings, graphics animations,... are memory hungry. And the DreamOS is not even using the Memory left, so don't worry the total memory is in good hands.
Here in a running DarkShadow installation (otherwise /data is almost empty as only a recivery image is there):
root@dm7080:~# mkdir /data
root@dm7080:~# mount -t ext4 /dev/mmcblk0p2 /data
root@dm7080:~# df -h
Filesystem Size Used Available Use% Mounted on
/dev/root 937.0M 348.1M 541.4M 39% /
....
/dev/mmcblk0p2 2.6G 435.0M 2.1G 17% /data