Webster left something out here. Yes, a line is rhopalic when each word or group of words has one more syllable than the next. But verse can also be rhopalic when each line is a foot (or so) longer than the previous. Rhopalic verse is also known as wedge verse, or snowball verse.
The word rhopalic comes from the Greek rhopalos, for a club. Think of how the cartoon caveman's typical weapon priapicly thickens.
A classic example of rhopalic verse is Metaphysical Poet Richard Crashaw's Wishes to His Supposed Mistress, which contains the lines:
Who'er she be
That not impossible she
That shall command my heart and me;
Dmitri Borgmann came up with a perfect example of a rhopalic sentence:
I do not know where family doctors acquired illegibly perplexing handwriting; nevertheless, extraordinary pharmaceutical intellectuality, counterbalancing indecipherability, transcendentalises intercommunications' incomprehensibleness.