If the problem was primarily due to the biscuit swelling, you would not have a perfect outline of a biscuit telegraphing to the surface. The wood fibers of the surrounding substrate would distort this more. If you still doubt this, soak a biscuit in water, wipe off any excess water, and then insert it into a slot (no glue, but you'll need a hammer). Immediately sand the surface of the wood before any moisture from the biscuit transfers to the substrate, and then examine the results in a few days. You may find a depression, but it will not precisely mimick the outline of the biscuit. The depression, if any, will be much more gradual and bowl shaped.
I am not saying that the biscuit swelling does not contribute to the problem, but it is not the primary problem. It is the excess glue sitting in the slot, and this will be absorbed into the substrate.
Oh, I almost forgot. Since expansion due to moisture is a ratio of the material thickness, the 4 mm thick biscuit does not change very much in thickness compared to the thickness of the substrate.
Rick, I appreciate the direct answer. I still doubt the findings...and I guess more importantly, does it matter? With biscuits that are made compressed, they have a chance of swelling the joint and causing some disfigurement of the final surface. I do not see that happening with the Domino and definetly have not had any such occurrence.