Assuming I don't screw up, I get about 85% mash/lauter efficiency, but only 75% brew house efficiency. At least some of this drop is caused by dead space and wort trapped in the equipment, and this will be a constant irrespective of the batch size. This would increase your brew house efficiency a little bit for a larger batch, but I don't think it would cause a 15% jump.
Do you fly sparge? If so, the increased grain bed depth could cause a noticeable increase in efficiency, and no matter what sparge method you use, the dead space in the MLT will be a constant which will increase your efficiency a bit with the larger batch.
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