Hamlet is replying to the King's statement:
But now my cousin Hamlet, and my son --
Hamlet (aside) A little more then kin, and less than kind.
Claudius How is it that the clouds still hang on thee?
Hamlet Not so, my lord. I am too much i' the sun.
There is at least a double meaning to Hamlet's reply. One is that he dislikes having Claudius constantly referring him as his son, since he obviously does not like being forced to have more than one father--especially while he is still mourning his first one. He also means that he is getting too much exposure, too much royal attention. He is being forced to attend this big reception party because Claudius wants to make it appear as if Hamlet approves of being cheated out of his birthright to inherit his father's throne and approves of his mother's marriage to his uncle, who is now both his uncle and his father. Hamlet wants to return to school at Wittenberg, but Claudius won't let him go because he is fearful and suspicious of his moody stepson and wants to be able to keep him under surveillance.
Hamlet may also be implying that he has nothing to do but sit around outdoors, or perhaps go riding, because he can't study and has no useful function to perform at the castle. In this respect he resembles Orlando in Shakespeare's As You Like It, who complains to his elder brother that he is kept in ignorance, dependence and poverty when he should be improving himself through study or useful employment.