Not all long range aircrafts are big plane. Several business jets have ranges over 6000 nm. However, they are not for average consumers.
The long range aircrafts for airline use are all designed as big aircrafts based mostly on economic reasons, as you can imagine. The airlines will meet with Boeing or Airbus and ask the question: based on my capital budget, the appreciation cost, the fuel cost, labor cost, etc, how much passengers and cargo I need to fly from point A to point B to be profitable. Now also factor in world economic outlook, consumer demand, current technology, etc, what should be the size of the aircraft?
It's technically possible to fill the cabin and the cargo hold of a 737 with fuel tanks and make it a long range aircraft. But what's it for other than ferry it from point A to Point B?