If planes are full, how can it be more efficient to fly my ~200 lbs (including lots of luggage!) from point A to B to C, adding extra distance and another takeoff, rather than directly from A to C?
The short answer seems to be no. Newer planes are much more efficient than older ones, but the most efficient aircraft and flight segment per passenger-distance are the A320 Neo and 737 Max making ~1000 mile flights.
Fuel burn is probably not the main reason airlines use the routing that they do.
I'd guess that staffing and gate time are big cost drivers. A lot of those "how Southwest succeeded" articles from back in the day cited very fast gate turnarounds as being a key thing— get the people off, get the other people on, get back in the air.
Are widebody planes that much more efficient?