Bigger PSUs are often recommended to account for adding a second GPU in the future, not to mention overclocking. Using the 5770 as an example, a single card would run on a 450W unit at maximum efficiency, with room for overclocking/adding other devices. However, once you add the second one, you'd want at least 650W to have a comfortable cushion, especially considering the 600W units (which is more like the requirement) are typically lower quality.
Besides, 200% of the power needed (double the required wattage) means the PSU is operating at 50% load. That's the most efficient range for a PSU. I wouldn't say the efficiency is lousy below/above that. It's usually only 2% lower, assuming the units have an 80+ Certification (which they always should).
I doubt anyone has recommended a PSU that produces TRIPLE (300%) what's needed. Using the same 5770, a 400W unit is needed (according to ATI), so that would mean people are recommending 1200W units. I've NEVER seen anyone that actually offers good advice here going over 850W. In fact, most everyone here will make fun of people who continually recommend that large of a unit.