I've been thinking about this some more...
I feel that the reason for the change has more to do with how the feet look when they have sandals on, rather than when they are bare. The old style slip-on sandals which were squarer did tend to make the klicky's feet look clumsy and overly large.
They must have been trying to make the new figures retain a streamlined look both when wearing shoes and without them, since many of the newer barefooted klickies in the Freetime range come with footwear.