For normal driving, the engine is just loafing along and not making much heat so the radiator size is not too crucial. It becomes crucial when the engine is asked to do some work and starts generating a lot of heat.
That style of shroud is very inefficient and does not let much air get pulled thru the entire core so the radiator is effectively much smaller than the actual core area would suggest.
Next, we find that many aftermarket electric fans are vastly inferior to the factory mechanical fan. Some factory electric fans like the HHR, the Lincoln, etc. do pull a lot more air than do many of the aftermarket fans unless one has a high dollar fan like a Spal.
Then, as Brad suggested, some aftermarket pulley sets that use underdrive pulleys may not move enough water.
And finally, there have been discussions here about certain aftermarket water pumps and/or housings (depending upon which thread you believe) that don't flow properly and overheating occurs.
If you start the engine and let it idle on a hot day, does it get hot?
Or, does it get hot when cruising around and then hit stop and go traffic?
Or does it get hot after a few wot bursts and then you slow down/get into stop and go traffic?
