In Chapter 3 of Freak the Mighty, Max says that Kevin scared him, but not really. What does he mean?

Here's the quote you're referring to:

Big goon like you, growing about an inch a day, and this midget kid, this crippled little humanoid, he actually scared you. Not the kind of scare that makes your knee bones feel like water, more the kind of scare where you go whoa! I don't understand this, I don't get it, what's going on?

Max has grown up with everyone around him that is close to his own age being afraid of him, or at least intimidated by him. When he sees Kevin moving in next door, he is first shocked by his appearance. Max says Kevin's head is normal size, but his body is twisted and only about as tall as a yardstick. Yet this tiny boy turns, looks Max straight in the eye, points a crutch at him, and yells at him! Max has never had a peer confront him before, and the first one who does is the smallest person he has ever seen. This flips Max's world upside down. He has hidden behind his exceptional size to keep others from getting too emotionally close to him. Now he has a new next-door neighbor who is fascinating and not intimidated by him in the least. Although Max is afraid of making a new friend, he is also very excited.