To understand tumid, think about how your belly feels after Thanksgiving when you've just polished off that third helping of turkey with stuffing and Brussels sprouts. Gassy? Full? Distended? That's tumid for you.
Tumid is also used by critics all over to describe their subjects (or rather "victims"). Any art form, particularly writing or acting, is ripe for description as tumid if it's overblown, bombastic and, well, gassy. Often used interchangeably with turgid in this sense.