With that 55 million trade, similar to the 72 million trade

With that 55 million trade, similar to the 72 million trade on September 22nd...we hardly see a budge of the PPS going up or down. Why is it that a large volume barely moves the PPS, but when you see a small tiny volume...it changes the pps in a big way?

Anyway, 55 million was quite a volume today.