Why are THP 1/2W resistors bigger than 1/4W while 1206 resistors are all equal?
SMD 1206 defines the flat/projected size of the component / in other words, it defines the footprint.  But they do have differences when their wattage is different.

How much heat the component can handle not only depends on size, it also depends on other factors: component matérials used, board layout, ... .

I could find SMD 1206 resistors that can handle 2W.  The manufacturer indicates that the actual power depends on how the device is actually mounted.  The 2W device has an aluminium core which helps conducting the heat to the board and possibly a heat sink.
The SMD resistor is directly on the PCB so it is "easier" to transfer heat from the resistor to the PCB than it is for the THT resistor.  In addition, the PCB layout can be designed to improve heat dissipation.

There is of course also the issue of pricing - when you are using THT you typically care less about space occupation, and increasing the size of the THT resistor to cope with double the power, is a relatively small increase in the 2D space taken on the board. (The axial resistor is cooled by the air flow, unless you put a heatsink on it using a clamp for instance).