I'm not sure it's "working harder," so much as working without a break from responsibility. An office worker, or even a factory worker, gets a break. He can go to the bathroom on his own. He gets a lunch break. He can take his eye off his coworkers, and not have them all disappear outside into a mud puddle. He can take sick leave if he's sick, and go to the doctor *by himself*. If he has to go on a business trip, or just to the gas station, he's only responsible for himself.
My husband once said, "Well, I'm responsible for the guys who work for me!" and I said, "You have to take them to the bathroom? Do they have clean clothes, if you don't do their laundry? Do they eat meals that you don't fix? What if they wake up in the middle of the night ... do you have to figure out their problems and get them back to bed? Wow, and these guys are earning a salary?!"
This is not to say that I feel oppressed about my life as a homeschooling mother of seven ... I wouldn't do it, if I weren't content. Just pointing out that to me, what makes it "hard" is not teaching phonics, doing laundry, or cooking meals, but rather the *responsibility* that you never get away from.
I think that is where the term 'Ball and Chain' originated from...(Ducking for cover). At any rate, I think that is the hardest part about having kids, you can't find time to just sit down and relax (unless they fall asleep). If you don't watch out for them they will injure themselves, etc. If one defines work as responsibility, then one can see a woman at home with 7 kids out works most people easily.
The other side of the coin is time - If someone marries and has a kid at 20 and the woman opts to stay home then she can semi-retire at 38 when the child grows up and moves on whereas the male in that scenario will still have almost 30 yrs more work (and again, this is a generalization, sometimes they are more problems when 18 then when they are 3).
My husband works long hours at Fed Ex. I told him once that when he comes home it's done. He doesn't have packages chasing after him asking questions or calling his name.