What about the possibility that the magic of the tunnel meets the intruder's expectations? When Harry and Hermione first enter, they expect a dark narrow hole in the ground and that's what they get. When Sirius and Harry leave again, they are so deep in conversation that they don't think about the tunnel and it let's them pass easily. The same with the Marauders, they knew it's large enough to let them through and so it did and they might even have gotten more confident each time, making the tunnel broader and the ceiling higher.
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Date: 2012-08-05 06:47 am (UTC)When Harry and Hermione first enter, they expect a dark narrow hole in the ground and that's what they get. When Sirius and Harry leave again, they are so deep in conversation that they don't think about the tunnel and it let's them pass easily.
The same with the Marauders, they knew it's large enough to let them through and so it did and they might even have gotten more confident each time, making the tunnel broader and the ceiling higher.