Since I have small (but very wide) feet I've never had that problem.
I've found most sailors don't really exert the necessary force to pull
the inner strap fully around the metal wire bail, so they give up and
cannot open the straps enough.
I loosen the inner strap, then put both knees on the board and pull
the center of the strap up with practically all my strength.
Then they are a bit loose and I can cinch them down for a really
nice fit that allows my heel to be placed in different positions along
the rail to achieve different weighting an help speed and handling of
The gentleman who owns Starboard has fairly narrow feet, so he likes
all the boards to have straps that are set really close together (too close
for my much shorter but wider feet).
I've found that spreading the mounting screws out at least one hole makes
them fit my wide feet nicely.
Your mileage.... and adjustments.... may vary.
Hope this helps,