The more I think about it, the more the shape of the blocks themselves comes into play.

These blocks all had rounded corners. They were, apparently, intentionally rounded.

Imagine the the size and weight of these monstrosities, barreling down the passages. The rounded edges would make sure (especially along the front four edges) that they would be past anything before they could catch on it. The fact that the lowest block had a significantly smaller leading edge, would make it work as a "plow" of sorts.

As gravity pulled these blocks down the passage, if there were any side walls that were too narrow, they would be 'thrust' back, out of the way, by the force of the wedge-shaped block at the lead. The only places this would not happen would be in the girdle stones, where the entire shape of the passage is "carved out" of two blocks stacked one atop the other, as a canal of sorts. Any other blocks, however, that were too narrow, could easily have been shoved back into the core masonry (and there are some pretty large gaps in this masonry to absorb such a shifting).

Also, we are looking at these passages after millenia of settling and seismic activity. These narrow spots (or short spots) may simply be the result of blocks shifting in to the empty passage. Nature abhors a vacuum.

Just more thoughts on the subject.

Anthony

You can lead a horse to water but you can't make him think.