If I worked it out right, 7kg of grain would displace 4.2L of volume. So if you mash with 3L:1kg, 7kg of grain will take up 25-26L of volume.
Is the absorption of water a factor?
EDIT: Wait, is it 11.66L for 7kg of grain? So 21L of water, 7kg of grain, and the volume would be up to 33L?