Most experienced developers are leaving Samsung for manufacturers that don't treat them like **** (see the Superbrick fiasco), leaving only the "me-too" smali-hack-kangs and a very small number of experienced competent developers like Andrei.
Quality, not quantity - most of the stuff in the I9100 and I9300 forums is crap, people who took a Touchwizz base, themed it, made some build.prop hacks, and called it a whole new firmware package. They're lucky in that Touchwizz is so bad that it's easy to improve upon it.
Compare to, for example, the Xperia T where there are 5-10 or more developers all contributing as a team to something that only has two threads (One for CM10, one for CM10.1) There aren't many people working with Sony stock firmwares because, honestly, it's REALLY hard to improve upon them because they're so good to begin with. Most of the few things that can be improved are achievable with simple TiBu freezes, no smali hacking required.