Consider Volpone as a comedy of Humours
Following is an attempt to give you a satisfactory reply:
In ancient & medieval medicine, it was believed that the four basic fluids of the body (blood, phlegm, choler/yellow bile, melancholy/black bile) directly affect a person's physical condition, and these fluids were called humors. When the humors are in a balanced state, the person will remain in a good temper. If, any of the humors gets imbalanced, the person also becomes abnormal physically & psychologically. For example, dominance of blood makes human sanguine (happy, generous), phlegm makes human phlegmatic (cowardly, pale), choler makes choleric (hot tempered, impatient, vindictive), black bile makes pensive, sentimental, melancholic.
The conception of the humor of the medieval age has a great impact on Ben Jonson's Volpone, especially on the character development. Actually, Volpone is, to many extents, based upon the humor theory. That's why it is considered as a comedy of humor. Jonson tries to show using the above notion that, if somebody lacks any character trait or contains too much of that particular trait, s/he is considered to be abnormal because of her/his imbalanced features. If a person does contain all the traits in a measured or balanced state, then s/he can be considered normal.
In Volpone, Volpone, Mosca, Corvino, Corbaccio, Voltore, Sir Politic-Would-Be & his wife, all of them do have imbalance in their characters, and this imbalance make the play a comedy since these abnormal characters pave the way to make the plot satiric & at a time amusing. Each character is peculiar & singular in his/her own way. It is necessary to keep in mind that, a comedy of humor always deals with the characters more than anything else.
Jonson indirectly hints that, the mental imbalance is more dangerous than physical imbalance as he shows that, the characters - Nano, Castrone & Androgyno - being physically abnormal, are better creatures than the earlier ones.
(Reference: Kathleen Morner & Ralph Rausch, NTC's Dictionary of Literary Terms)