Power and weight

I am in middle of a racing series, and analyzing the results reveals that the zwift algorithm appears a bit hard for heavier riders. I have completed races riding in small group from start to finish, giving me an opportunity to compare the results. I am heavier than most other riders in the group, and (not surprisingly) I need more watts for the same end time. What is surprising though is how much more watts I require. I did some testing in Zwift to find out that on a 0% at 35kmh, a 10kg heavier rider needs about 13 watts more, while when climbing at 7%, a 10kg heavier rider at 15 kmh needs about 55 watts more. Hence, on a flat course, I would expect to average about, say, 15w more than the lighter riders. In practice, however, I need about 30 - 50w more. Any ideas?

Could there be a difference in where you measure the power?
Pedals measure before any drive train losses and trainers measure at the end of the line.
Also, is your model correct?
How do we know Zwift is incorrect or in error.

Zwift is not a Real Life Simulator.
It is what it is and it is most definitely different than IRL because real life has a lot of factors.

Did you compare heights along with weights?

deleted, on 2nd thought, can’t be bothered by the ensuing CP discussion

Compare the numbers in a TT where things aren’t muddled by drafting behavior.

The way pack dynamics work currently, a lot of riders are working harder than they need to in order to maintain position. In other words they are not working hard enough to move ahead, but they are working harder than they need to in order to not move backwards. The canny riders do this properly. Most do not.

Good point, and I have been doing my best to benefit of drafting, so I’d like to think that drafting does not make a the difference here.

Andreas, thanks for you reply, very interesting. I agree that heavier riders should produce more watts, and hence have an advantage (at least) on flat. My own zwift testing (by manipulating my weight) shows that by reducing my weight 85 → 75 I need (roughly) 13 watts less on flat. However, as many riders in the race I was referring to averaged 3.2 to finish the same time, it means I needed 32 more watts for the same result. If the race would have been on flat, 13 watts should have been enough. Simple explanation would be that the race was not on a flat course, and therefore I needed more for the same result. Actually, by analyzing my test results a bit more (85kg rider needs about 55 watts more than 75kg rider to climb 7% at 15 kmh) reveals, that if 85 kg rider and 75kg rider both ride at 3.2 wks, they should finish the same time, if the uphill is about 4%.