Well, I think they are not... (harder for your legs)
You ride with more speed to get the lift, but not more power or pressure really.
When getting used to it, you WILL of course be much more tired, but when used to it, no difference.
I got wings at both ends (medium area high camber wave wing, small area low camber fast wing), so above is my experience.
Having said that, if you go for upwind, yes, you will most likely want to ride with more power from the kite, thus more leg workout.
But will still say, most of it is getting used to the difference