While it might feel like your a are burning a ton of calories while you are riding at a hard effort
it does not compare to a sport like running.
I have heart rate monitor and it is hard for me to maintain 140+ on a bike, 155 and I am really feeling it.
When I roller blade I hit 152+ and it feels like I am hardly trying, and I can maintain it for hours.
This is gbg's experience. It is not universally the case.
You need to bulk up your leg muscles if that's how it is for you. Also, try toeclips and straps or clipless, so you could recruit more muscles.