I'd just like to point out in a terribly sad fanboyish way that Rise says her breasts are actually quite small whereas Yukiko's breasts can be pretty big in some official fan art. Therefore it'd probably go:
1. Naoto
2. Yukiko
3. Chie
3. Rise
5. Nanako

(The joint 3rd is intentional, not a typo).

I seem to recall Rise saying that her bust was 82cm (IIRC) and called Yukiko's "medium" sized (saying that "don't worry, plenty of guys like that") and while it's pretty much canon that Naoto is the largest, I think THIS might be a bit of an exaggeration. Dare I ask how much tape she needs to go from THIS to flat? And her poor internal organs...

It's an exaggeration for sure but I'd guess the proportions are about right between the girls. Wonder where Nanako will sit in the rankings later on.

I'm just sad Yukiko isn't wearing anything particularly revealing, dammit.

Sorry? I missed what you were saying. I was enamored by "small".

Well yukiko has to be bigger than chie

I'm pretty sure you're right, Chaoticsouls.

Whoever drew this just has obvious preferences. Rise's DEFINITELY not this big.

The game never actually says that Naoto's are the largest, Rise just reads her measurements on a medical chart and gives the reaction:
"Wha-?! That's-That's not possible!"
So it's just generally assumed that Naoto's are big, and she binds them to keep up the detective prince thing...