For work on a circle the horse actually needs to be straight
on the circle, so if you were looking from above, his outside and inside legs would be describing concentric circles, but the outside one would be bigger than the inside one. One thing to check for is that the inside hind (being the one you can generally see best

) stays true to its own circle and doesn't cross under as it comes forward.
Can I follow up for clarification?
For the sake of argument, let's say we have a horse who is tracking up right into his front hoofprints when travelling straight.
Now put him on a circle. So if the outside part of the circle is bigger, would you expect the outside hind to reach/stretch a greater distance than the inside hind leg, so they both drop into the front hoofprints (so one leg stretches/reaches more than the other)?? Or would you expect the two hind legs to reach the same distance as one another, so the outside leg lands notably behind the outside front hoofprint relative to the inside hind leg which might still be able to land in the front hoofprint??