You're right the fuselage dimensions have remained very nearly the same, except for length, since the 707 days. The -700 is equivalent to the -300, the -800 to the -400, and the -600 is close to the -100 in size. The materials and assembly methods are better, however. For one thing the fuselage is assembled in one piece in Wichita and delivered by rail to Renton. (Looks funny to see a complete fuselage going by the windows in my favorite restaurant in Parkville MO
every so often.) The -900 and -900X rival the 757 in seating capacity.
is using a FADEC (Full Authority Digital Electronic Control [Fly By Wire]) engine, the CFM56-7B, where the classic used the -3 which was primarily cable controlled.
The wing is still a 25deg. sweep but with a deeper dihedral. The flaps have been completely redesigned and are more similar to those on a 757 than the old 737 flaps. The leading edges are still very much the same but the 900X will have sealed slats. All the NG
's have one more slat per side than the classics.
The horizontal stabilizer and elevators are much larger and longer as is the vertical stabilizer and rudder. The vertical is also a complete redesign and because of its size the yaw damper only has 2-3deg movement needed to take care of any Dutch Roll tendencies.
's do carry more fuel in basic configuration but that's due to the wing redesign and a larger center tank. The range is more a function of engine improvements and efficiency. The -400 had a range which would allow Boston to Salt Lake or Keflavik, ETOPS wasn't really a consideration. The -800 can fly Boston to Madrid/Paris/London or Los Angeles without Aux tanks. The NG
's are delivered ETOPS certified for 1 or 2 hours depending on if the customer wants one or two batteries and cargo fire bottles.
The plane might look and operate, from a crew point of view, like a classic, but from the maintenance point of view they have to be taught completely separately because they aren't the same.
Looks can be deceiving
One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533