I believe that you are referring to Max Kane and Kevin (Freak) from Philbrick's book "Freak the Mighty."
One similarity between the two boys is that they are both judged by their outer appearance. Max is a big kid. Really big, but that's not the main physical characteristic. Max looks just like his dad . . . the dad that murdered his mother. Everywhere Max goes he feels people making that comparison. Freak is also judged by his looks. He's freakishly small. Like 3 feet tall and has to walk around on crutches. Despite people's best intentions, it would be almost impossible to not make a snap judgment about Freak based on that. Max even does it. His initial description of him is a "crippled-up yellow haired midget."
On a "deeper" level, both Max and Kevin are broken characters. Kevin has serious health issues and eventually dies because of them. Max is broken on an emotional/psychological level. He witnessed his father kill his mother, and Max has retreated from everything since then. He distances himself from family. He has no real friends (except Freak). He's dead on the inside for quite a bit in the novel.
As for their differences. That's a bit easier. Freak is tiny; Max is huge. Freak is crazy smart. Smart on the level of genius. Max isn't that smart and knows it about himself. He calls himself dumb and brainless on more than one occasion. That's surface level stuff though. The real difference between Kevin and Max though is their outlook on life. Kevin may be dying, but he loves life. He has an attitude of carpe diem (seize the day) about everything. He's always making up some cool adventure for him and Max, and he does it whether or not Max is in the mood or not. Without Freak, Max would almost never leave the underground he calls home. Max wants to distance himself from everyone and everything, and if it was up to Max, he'd probably never go out.