Basically, Mikuru asked Kyon what boys do when they suddenly get a hard-on. Given that Mikuru keeps spacing out and not noticing it, then accidentally smacking it into desks and tables, this leads Kyon to speculate about whether or not Mukuru's size 'down there' is somehow related to her previous breast size.
Getting back on track, Kyon explains that to avoid 'accidents,' Mukuru could 'do it by hand' before school, or in the bathroom, but Mikuru didn't understand that euphemism. Koizumi then arrived, which led in this page to Kyon speculating that Koizumi's breasts are really huge, maybe even bigger than Mikuru's.
In the really text-filled panel in the middle, Kyon's getting carried away with her previous train of thought, wondering if Koizumi's breast size might be related to how 'big' he was before - and what that says about _her_, since Kyon is totally flat-chested. In the end (probably to make herself feel better) she reasons that there must be no relation, and it must all be about what sort of 'image' Haruhi has of them.
It's okay. I can't read jack, so a summary of the scene is at least more than I could understand before.
But nevermind me.After all that, what does "tossing off" mean?Or again, the sinful act of masturbation.Koizumi.Is it ok not to be disguising yourself as a guy?That form has completely gone.In the period after class is dismissed, as a North High first-year schoolgirl, I am humbly being forced into activities.Suzumiya-san has also acknowledged this fact.You went up and talked to Haruhi? Hah, that's a weird story.Ah, that guy seriously has big breasts.Could they be bigger than Asahina-san's...No, wait?If Asahina-san's presence in her breasts were transferred unchanged to her nether regions...Then the force in Koizumi's chest... in short, we could conclude that as a guy--No, stop. WAIT.That would mean, in other words, because it's the same size type as my chest, my THAT would fall a great deal below the average for men...No no no no. No. I think it's wrong, "this" was, in other words, primarily the image that Haruhi had of us. That idiot Haruhi, in other words, was thinking that as a guy I was this sort of size. That fucker, I'm going to punch her someday.Ummm...I see!That's what you were talking about?What does it mean? That expression.