Usually masters classes are a much larger volume of information coming at you in a shorter period of time.
On SDN people always cut down grad classes as having grade inflation but that is not really the case. In grad school everyone is there to work and has gotten over their immature-not-going-to-do-work phase so instead of having a bunch of slackers like you would in regular college, they have all been weeded out.
While I did better in grad school than in college, I though it was more difficult.