Do you coast/soft pedal a lot more outside versus inside?
Many people severely overestimate the work they do outside. Without a poweremeter and an intent to maximize time applying power to the pedals, it's very easy to spend 30-50% of your time coasting or soft pedaling outdoors, especially in rolling terrain.
And if that's the case and you pedal the entire time while indoors, then it can certainly seem tougher.