I'm not sure if it's actually wrong, but rather a function of how they draw atoms versus molecules.
Diameter is a funny concept for atoms and molecules because electrons orbit at different distances around the nucleus probabilistically. The depiction of this carbon atom I think represents this, with the lighter color areas denoting areas of lower probability.
However, drawing such a diagram for a molecule isn't very pretty, because the orbitals for each atom in the molecule overlap. They probably drew the water molecule using intermolecular distances, and then filled in the distances so that each atom was a sphere. In actuality the electrons are found with some probability outside this area.
Actually, as I look at carbon more closely, that's exactly what they're drawing. The inner circle (s orbital) has two yellow dots (electrons) on it, and the outer circle (p orbital) has 4 yellow dots. It does look like they totally ripped off this lady's drawing though: http://www.dorlingkindersley-uk.co.uk/nf/ClipArt/Image/0,,_1...
Clearly they aren't drawing the orbitals for water, which I already stated.
But you're right, carbon is probably too big. Wolfram alpha says the diameter of carbon is 134pm, and we all know WA is right about everything.
The sizes of the atoms in the water molecule have been adjusted to
make them consistent with other atoms depicted in the activity,
although the overall diameter of the molecule is the same.
Diameter is a funny concept for atoms and molecules because electrons orbit at different distances around the nucleus probabilistically. The depiction of this carbon atom I think represents this, with the lighter color areas denoting areas of lower probability.
However, drawing such a diagram for a molecule isn't very pretty, because the orbitals for each atom in the molecule overlap. They probably drew the water molecule using intermolecular distances, and then filled in the distances so that each atom was a sphere. In actuality the electrons are found with some probability outside this area.