Shoushitsu Koizumi was pretty straight, as I recall. I don't remember him being particularly interested in Kyon. He even seems to dislike him and with good reason. Regular Koizumi shows the same interest in Haruhi, at times, but avoids it.
She's always been stacked, but she's in the same series as Mikuru so she seems smaller then she really is. That and her chest isn't as emphasized as much as Mikuru's is, again leading to the appearance that it's smaller then it really is.
