This will not be an in-depth answer as I am unable to do cite sources at the moment, but as I recall this was based on a decision to reduce production (printing and materials) costs, hence the new notes being approximately 30% smaller than previous notes. It was also an effective anti-counterfeiting measure, as there was a hodge-podge of various sizes, designs, and inks used for paper currency prior to the Series 1928 bills, which made counterfeiting easier and legitimate currency harder to distinguish by the general public. Having all paper currency the same size and color allowed for a single size and type of paper and plate to be used, and for green ink to be purchased in quantity.

As always, I defer to anyone with more detailed, expert knowledge on the subject.