Parodied: Regardless of age, race, and overall probability, every man is a lot taller than the women.
Zig Zagged: Bob looks taller than Alice in one shot, but he was standing on something, then Alice turns out to be a hologram by the very tiny Alice the Fairy, and Bob turns out to have been cursed to a smaller body, and then it turns out that Alice is a sizeshifter...
Height and biological sex are independent of each other.
For the Boys Love variant, height and role in the relationship are independent of each other.
The writer of the show believes in rigid gender roles, so the male partners are always depicted as taller as a visual analogue to the author's belief that men are/should be the more dominant sex.
Alternatively, the writer insists on carrying over said gender roles and stereotypes into homosexual relationships.
The actor playing Bob is somewhat taller than his home nation's average, so this trope is the result.
Lampshaded: "How come all the guys I meet are bigger than their girlfriends?"
Invoked: Bob attempts to somehow make himself taller in order to win over Alice.
Bob deliberately seeks out a girlfriend that doesn't care if he is shorter than them.
Bob deliberately seeks out an Uke that doesn't care if he is shorter than them.
Deconstructed: Bob has trouble finding a love interest because everyone he meets insists on this trope.