To offer a bit of debate:
240 radiator over a 120 makes a rather considerable difference, actually, but ultimately comes down to thermal load at peak usage.
Push pull also does improve cooling - radiators and heatsinks cause dissipation and resistance that 1 fan often struggles to force air through. If you have enough depth, this is the ideal fan setup for almost every radiator, however normally chassis space is limited, so only 1 set of fans is typically used, also because this is the cheapest solution.
The fans are still what is ultimately cooling your setup, because water isn't very good at dissipation.
Correct, which is why a 240 radiator is often beneficial over a 120, especially over 150w TDP (overclocked or not). Liquids such as water and water-based coolants, like glycol mixes typically used in AIO coolers, are able to absorb significant volumes of thermal loads before the actual CPU begins to see temperature increases. Likewise, this is also the same reason that liquid cooling radiators often remain warm for several minutes after load ceases. If you do not have enough liquid volume to absorb the entirety of the thermal load while also being able to dissipate a relatively equal delta of thermal volume simultaneously, the cooler theoretically will continue to absorb more and more heat, getting warmer and warmer.
Think of a boat - almost every boat leaks or takes on some water but also has pumps to discharge it back into the ocean or lake. If the boat takes on more water than it can pump out...well, you get the idea.