Pay load is one of the weakness of Mig-21 Fishbed.  Mig-23 seems to me like a lesson drawed from that.  Mig-23 is much bigger and has much more payload than Fishbed.  The air aircraft maximum range is 1900 km (slightly over Mig-21) while it is even fly faster than Mig-21.

The Flogger B is a standard interceptor. The aircraft is in widespread use in Eastern Europe and the Middle East. Other versions of this aircraft are: C--two seater; G--improved interceptor; and E--export.  The wings are high-mounted, variable, swept-back, and tapered with blunt tips. There is one turbofan engine inside the body. There are rectangular, box-like air intakes forward of the wing roots and a single exhaust. The fuselage is long and tubular, except where intakes give a box-like appearance. It has a long, pointed nose and a stepped canopy. There is a large, swept-back, and tapered belly fin under the rear section. The tail is swept back, has a tapered tail fin, has a curved dorsal in the leading edge and an angular tip. Swept back, tapered flats have angular tips and are high-mounted on the fuselage.
 
 
 

Technical information:

Manufacturer                                          Mikoyan Gurevich OKB
Designation                                            MiG-23
Nickname                                               Flogger
Type                                                        Front-line variable swept wing fighter
Crew                                                        1 pilot
Length (with probes)                             17.05 m
Height                                                     4.35 m
Wingspan (min sweep angle)              13.8 m
Wingspan (max sweep angle)             7.4 m
Wingarea (min sweep angle)               37.3 Sq m
Wingarea (max sweep angle)              24.2 Sq m
Max T-O Weight                                     17900 kg
No. of Engines                                        1
Powerplant                                               Tumansky R27F2M-300 turbojet
Thrust (each)                                           10200 kgf (22467 lb)
Max range                                                1900km
Max Speed at sea level                         1350 km/h
Max Speed at high altitude                   2500 km/h
Service Ceiling                                        18000 m (59016 ft)
 
 

Two Mig-23 taking off