I've heard that you get more extraction from hops in big batches, but I have no experience to say if this is true or not.
From ProMash help:
Kettle Size Hop Utilization Scale Factor
When scaling a home brew size recipe to micro-brew size, the micro-brewer may need to set a value here to reflect the increased hop utilization common to large batches and professional kettle geometry. Entering a value above 1.000 will result in higher IBU predictions in the stand-alone IBU Calculator as well as the IBU calculators imbedded in the "recipe" and "brewing sessions" formulation screens. Entering a value that is less than 1.000 will result in the converse being true.
As an example, the author has found that a value of 3.4 is appropriate in his brewery when scaling a 10 gallon recipe to a batch of 17 barrels. However, this may be completely different on your system, and we urge you experiment with a known recipe and "tweak" this number until the IBU values reflect accurate, known values.
Additionally, if you are trying to compare the IBU values generated in ProMash to another IBU calculator, keep in mind that very few (if any) take the "Kettle Size" factor into account, and the numbers will never match if the "Kettle Size Factor" is set to anything other than 1.0. The reason for the "Kettle Size Factor" is that most of the IBU formulas (with the exception of Tinseth) are curve fits and were based on small batches that home brewers use (Rager's formula is the perfect example of this), and really big, well designed kettles can simply utilize hop acids far beyond what is possible with small scale kettles.