The difference between undergraduate and graduate is a matter of degree, not a clear cut. The difference between first and third year students are as large (or larger) than undergraduates and graduates.

If anything, graduate classes are easier, the students take the courses because they are interested, not because they have to take it as part of the curriculum. You can tell people to study or find out stuff by themselves, you don't have to give them everything predigested. But they are harder, because you have to cover more material, and the material isn't readily available, is much less ordered and worked over, cleaned up and has not yet found its "optimal" exposition.