I would say Unicycling is much less effort.
If I have to pin it on a wheel size as an equivalent, I'd say a 20''.
This is provided that you can ride well and economically. When I started out, doing 1km would kill be, but with time, riding becomes much less of an effort.
On straights or slight downhills, uni is not really any effort, and you don't have to carry your weight like when running.
On steep uphills, sand or technical terrain, running is less effort.
I've ran over 100 races of 10km - 56km, and did about 15 cycling events (uni and MT
. I'm slow at both though - just going for the finish in time.