Half of those projects could be class libraries. You can't put those in different solutions cause you miss the whole point of having everything tied up.
We don't have hundreds of millions LoC, but we have a couple of million. We solve the problem you are describing with svn:externals, pulling in the DLL/PDB from other project/solutions as needed on svn up.
I agree it's not as comfy as having everything in one big solution, but being able to count your compile time in seconds as opposed to minutes makes it worthwhile.
