Barberry,
I think the difference in thickness between access doors in American silos and this Iranian silo come down to one thing: the missiles.
American silos today accomodate the Minuteman ICBM whose thrust is MUCH more powerful and MUCH hotter than the thrust from the Shahab-3 and therefore the access door for this Iranian silo doesn't need to be nearly as thick since the heat and stress it will have to deal with will be much less.
YMJ,
I was pondering that myself. Traditionally, the missiles are loaded via a track system (like a railroad) within the bunker and then using a hydraulic lift system to erect the missile into place. I don't see such a system from the images of the silo so far so I am not sure how they loaded..One hopes they didn't just drop it in from above..