weithman5 has an excellent point, and might be the biggest factor for 10-gallon batches or for certain systems. But for me, I actually find the opposite to be true -- I get better efficiency with SMALLER batches, based on a completely different effect -- Whether I'm making 2.5 gallons or 5 gallons, I am always boiling off a little over 1 gallon per hour. Boiloff rate is a constant assuming you use the same kettle for making both size batches. So as an example, percentage-wise, I am squeezing a lot more sugars out of my grain when I collect a pre-boil volume of 3.5 gallons of sweet wort to end up with 2.5 gallons post-boil, compared to when I collect 6 gallons to end up with 5 gallons after the boil. In both cases, I'll boil off a gallon, but percentage-wise, I know that I have to sparge a heck of a lot more on the smaller batches to get the final volume that I want. The result? On 2.5-gallon batches, I can easily hit 90% efficiency every time, boom boom boom. On 3.5-gallon batches (which yes I have done on occasion), it's more in the mid 80s. On 5-gallon batches, maybe the low 80s. It makes a difference, at least to a point. But eventually the effect might wear off with bigger batch sizes, or even out with the dead space effect. I've never made a 10-gallon batch, don't have the equipment for it, but it might make for an interesting experiment for someone else.