To put the above into math take 2 riders Rider A weighs 130lb, and produces 225 watts at his functional threshold. Rider B weighs 200lbs, and produces 300 watts at his functional threshold. On the flats, Rider A is good for 23mph, but Rider B can go almost 24, because he has more absolute power, and only slightly more frontal area.

But Rider A has a power to weight ratio of 3.8w/kg, and Rider B only has a power to weight ratio of 3.3w/kg. So on an 8% climb Rider A can sustain 8.2mph, while Rider B can only sustain 7.5mph.

This phenomena explains why I, as a big, relatively powerful rider can time trial reasonably well, and be a threat in flat races, but suffer on climbs.