Could this be a safety issue? Maybe, but not during a flight control check on the ground. If a persons size interferes with aircreaft control then yes and I would hope that it would have been identified during simulator training.
That being said if this was a TRUE safety issue for you then why did you continue the flight with this individual? Kind of like saying braking action is NIL then continuing to taxi. I feel that you are only interested in stirring the pot and embarrassing the Captain. I would be careful of such actions as this could be construed as workplace harassment.