Yes, smaller folks generate and sustain less power than larger equally fit folks and in general even when weight scaled women sustain lower power than similarly fit men.
For instance world class male professional riders can sustain up to ~6.4 w/kg for roughly an hour where the top world class women sustain roughly 5.7 w/kg for similar durations. In both cases that's an awful lot of power. But down in the amateur levels it's similar with a lot of fairly well trained but recreational men sustaining perhaps 3 w/kg for an hour and women a bit less around 2.7 w/kg. Those numbers are taken off of the power profiling charts you can find explanations for here:
http://home.trainingpeaks.com/articles/cycling/power-profiling.aspx
The chart isn't hard and fast but it gives you a general idea of the range of power both men and women sustain from untrained individuals up to world class.
In terms of watts to dietary Calories, take it in steps:
- The kilojoules of energy you burn when averaging a certain power for an hour equals average watts for that hour multiplied times 3.6 (as joules = watts*seconds and there's 3600 seconds per hour or 3.6 kiloseconds per hour). This part is straight physics definitions of watts and joules of energy and not subject to any personal differences between riders male or female.
- Technically there's 4.184 kj per dietary Calorie (or 4.184 joules per physics and engineering calorie but dietary Calories are 1000 physics calories, hence the capital 'C' in the dietary version). But that pretty much washes out because humans aren't that efficient in turning calories into productive work. A rider's individual Gross Metabolic Efficiency (GME) dictates efficiency in turning fuel combustion into useable work at the muscles of interest. GME ranges from around 19% to perhaps 27% across the human population and even pro level athletes see similar distributions. It takes a lab test to pin it down precisely but taking a swag that GME ~ 23.9% makes that conversion from kj to Calories wash out. IOW:
Calories burned cycling ~= average watts * hours * 3.6 * 4.184 * 23.9% = average watts*hours*3.6
So if you ride for an hour at 100 watts average power you'd burn ~ 360 Calories
Sustain that 100 watts for 2 hours and you'll burn roughly 720 Calories
Bump the intensity up to 150 watts and the burn rate climbs to 540 Calories per hour.
So yes, larger folks tend to burn Calories at a faster rate as they tend to sustain more power. Fitter riders also tend to burn at a faster rate as they sustain higher power, ride for longer or both.
Hope that makes sense,
-Dave