What can make a bike harder to pedal is heavy tires/tubes. When I bought my 26" fat bike (unpowered) , I took it out and could barely do a 2 mile ride, it was so hard to move. I added 14 pounds for motor/battery, but got rid of 12 pounds. much of it in the steel parts, including 2.4 pounds per wheel with lighter tires/tube. As a result I was able to do that same 2 mile ride with the power off. It's just easier to accelerate a lighter wheel. Nonetheless, fat tire bikes wouldn't be popular w/o motors. Only 100 of my 3000 miles ridden this year was on my fat tire bike. I should get rid of it.The little kids seem to like seeing it.
Rolling resistance is another factor, Getting away from fat tire bikes, my wife and I ride very similar 20" folding bikes and on hills, she will start 20-30 feet behind me and be passing me at the bottom. She's 40 pounds lighter, and sits upright. I lean forward, so think our aero resistance is comparable. I mounted 20x2" Schwalbe Big Apples on her bike, while I have 20x1.75" Kenda Kwests. Schwalbe calls its tire a balloon tire with surprising low rolling resistance. No lie. They roll. I just bought a set for my bike. By the way. little 20' wheels accelerate quick, both due to the gearing effect and the lower rotational mass. My 20" folder is easy to pedal on flat ground. Weight also matters. I consistently burn 30-40% more battery than my wife on the same ride, measured via wattmeter, so it's somewhat scientific.
If you ever tried to turn a geared hub motor axle with your fingers, you will feel quite a lot of resistance. But with a wheel/tire and mounted in a frame, it seems to spin easily, but not as freely or as long as the unpowered wheel, There's some drag, I also own a Bafang BBS02 middrive. I took off the chain and saw that the pedals would spin for about 10 seconds. No drag, but on a regular BB, the pedals will spin for over a minute. It's all little stuff. Adds up.