You need to make sure your heartrate monitors are set up correctly to start with. There should be a stage where you add info about
being male or female and also body weight. Once that's done, it should be accurate data if they're good quality monitors and the chest bands are fitted correctly.
It's likely that you're fitter, hence why he finds the same workout harder. The fitter you get the lower your heartrate will be to do the same thing. It's called adaptation and why to get more results you'll need to change something about
the workout, or do something different.
Unfortunately, being lighter you'll burn less calories in total than someone heavier. BUT you also need less calories each day, so the percentages may work out better.
If I run with my hubby and we run for an hour.. he might burn about
700 kcal and I'll burn less than 500. That said, he needs about
2500kcal per day and I only need 2000kcal. So it works out if you consider the percentages rather than the totals.
Just be happy that you're much fitter than him ;-) and find the same workout much easier, rather than worrying too much about
calorie burn
good job