Spin is just very intense. Over half an hours spin class i’ll burn anywhere between 375 and upto 500 calories. I get nowhere near that when running (not a good runner to be fair) or on a proper outdoor bike ride. It’s the interval nature of it, constantly changing the speed and intensity.
Remember your HRM is generally a pretty rubbish way of measuring calorie output.
Get fitter, HR drops, power output goes up (along with calories), but HRM says you actually burn less.
Assuming a 30min spin is equivalent to about 20min at threshold, and you’re a “good for a weekend warrior” 250W at threshold, that’s 300kJ of work done, or 300 calories of food burnt (calories of food in : work output in kJ is 1:1 because the efficiency and conversion are about the same).
You can’t cheat the system and burn more calories than your threshold power allows over a half hour session, for every interval, you have a rest, so it averages out.
The exception that proves the rule is that high intensity intervals continue to burn calories for hours/days afterwards as muscles need repairing.
