ahriman46,
Well for one thing, that is a Titan missile which has a diameter more than twice that of the Shahab-3 so the silo is bound to be larger on the inside.
Also, you have to remember US missile silos in theory are designed to survive practically anything short of a direct hit from a nuclear warhead.
For these reasons alone, the old Titan silos are going to be much larger on the inside with thicker walls and doors.
YMJ,
From what I know, all silos are designed to be almost completely self-contained. The missiles are often kept fully assembled in the launch tube unfuelled (in the case of liquid fuel rockets only of course) with warheads already attached. However that is in the case of nuclear-armed missiles.
But the missile bodies, warheads, and fuel are normally all contained in the same underground complex so that in times of war the missile silos can operate without the help of the outside, which gives them the greatest amount of survivability and increases readiness.
However, the point of having the entire process underground is not so much because of secrecy but because of practicality. Yes, you don't necessarily want the enemy to know if your silos have missiles in them or not or what their readiness is (though they will logically assume there are missiles in all of them) but with today's technology hiding silos is not so easy anymore.
For example, Synthetic Apperture Radars are capable of "seeing" under the surface, including in mountainous terrain. Now there are limits to how deep SAR can "see" underground but considering they were designed to be able to detect underground structures in the first place (along with being able to provide very accurate radar maps of rough terrain), it's likely the "stealth" quality of missile silos have faded a good deal in the last few decades as SAR technology has spread around the world.