Is there any correlation between the CPU and CPU candidates' oppai sizes like there is with KanColle?
I don't think there's a correlation, based on the fact that bust sizes change when in HDD form. Sometimes dramatically. Neptunia herself upgrades from a modest A to an E cup when she transforms into Purple Heart.

Before :



After:



That being said, I don't know much about the series as I'm only playing the first game.

Maybe someone else has more knowledge than me and can also answer. Here's a spreadsheetable list of some of the character's bust sizes for any other answer's research.