No, not every kid that gets bullied has a social disorder. However, plenty can develop one, especially if they're coming from backgrounds as crummy as Tobias's. I don't buy into the autism spectrum applying here either, as he shows a good amount of social competence, and I think his issues are 90% environmentally caused. What I meant was that there are certain ways to avoid bullying, simply in the manner you project yourself. Tobias didn't project himself at all, hoping he wouldn't be noticed, instead unfortunately leading the more sadistic classmates to see him as a weak, easy target.
David is a straight-up sociopath, not the least bit spectrummy. That's been analyzed to hell and back about half a million times, so I'm not going to get into it. Basically, the primary difference is that a person with Autism has difficulty grasping how to recognize and deal with the emotions of others and social interaction, a sociopath doesn't feel emotion (with some exceptions) and sees others as just objects, and a schizoid just wants to keep a distance between themselves and others and interact on their own terms. Naturally, like many other psychological conditions, these all have their own sliding scales of severity, they're not just stereotypical cookie-cutter molds.
As for the "one-sided" thing, I mostly meant later in the series, especially around #23 and after. Whenever they go on a date, it's always Rachel who set it up. When Tobias was starving out in the woods, he turned to roadkill rather than just stopping by and asking for something. Whenever he needed any kind of help, she practically had to twist his arm wing to get him to accept it. He still reciprocated emotionally and understood her like no one else. Like in #33, when he said,"Be Rachel," to tell her to show Taylor mercy. Coming from any other animorph, those words would have just meant,"Tear her damn cyborg head off." But no, he saw the struggle she was going through that no one else did (not even Cassie, her best friend and the supposed people-person of the group). He loves her, he gets her, but he's just a natural loner by preference, and didn't do as much to actively move the relationship forward as much as she did. If she hadn't taken up pretty much all the initiative, it would have just evaporated before Tobias even realized it was going. That's what I was saying.
The others all have numerous social venues open to them (though Cassie's vague in this department, but I'm saving her issues for a future thread), but Tobias is the only animorph specified in text to not have had any friends before this all went down. The implication is pretty clear there. I don't doubt he had playmates when he was younger. One of the flashbacks in #33 was on a trampoline, and his uncle didn't strike me as the type to go out and buy one for a kid he didn't even want to take care of. Socializing is far easier when you're a preschooler/elementary student. Keep in mind that very little is generally said of their lives outside the missions, and general school interactions are implied to have taken place.
90% isn't a guarantee, and even then, that only applies to normal kids. In fact, he shatters the mold with the sheer amount of bad stuff happening to him. By the end of the series, he's an emotional wreck and shunning nearly every bit of human contact. Apart from Rachel, he basically had very little hope for coming out of his shell from the start.
Also, Tobias wasn't chubby. ',:-o He was actually described as scrawny, if I remember.