I know you said that you used the default 5Gal equipment profile, but which one? In my copy of BeerSmith I have a few different default 5 Gallon profiles.
When I look at the "Pot and Cooler (5 Gal/19L)" profile in my software, which I don't think I ever modified, the Mash Tun Volume is set as 10 gallons.
I plugged in 4 Kg of Two row, and I got the following mash volumes which were not that different from your volumes:
Protein - 7.94 qt
Sacch - 7.05 qt
Mash Out - 6.17 qt
Can you post a screen shot of your Equipment profile?