Why density of states of graphene ribbon with PBC depends on ribbon dimensions?

[qweasel]

I used VNL to calculate DoS of graphene for two cases:
(1) One unit cell, with periodic boundary conditions,
(2) 4x4 ribbon (case (1) repeated 4 times in 2 directions), with periodic boundary conditions.

The resulting DoS charts are very similar qualitatively, the major difference being a 20x higher DoS for the ribbon case.
Why is this the case if I'm using PBC? Why the DoS changes with the size of the unit cell?

[Anders Blom]

Because you repeated it...

[Jess Wellendorff]

Your repeated unit cell contains more of the same atoms, and therefore also more electrons, so the density of electron states in energy-space increases.

[quantumtoday]

To make a fair comparison, the DOSs should be divided for number of unit cells  in each calculation (to have DOSs/unit cell) before to compare them