How does one extrapolate riding calories to fellow riders?
No, calorie burn does not scale linearly and can vary based on a great many factors.
Calorie consumption is very closely tied to power output.  Power in watts x4 is usually considered a good estimate.
If the entire ride was on a steep climb using tyres with virtually no rolling resistance, then in this case you would be correct.  However on a real ride other factors come into play (rolling resistance and aerodynamic drag).
In terms of rolling resistance, there is a large variance between tyres - or even between the same tyres at a different pressure.  This will also change (but not linearly) with rider weight.
Increases in body mass also do not lead to a linear increase in aerodynamic drag.  Further to this, power output can change a lot due to a riders position and clothing.