Max makes Gwen nervous because he is very big in comparison to her son Kevin, and because he looks very much like his father, who is a murderer.
When Kevin and Gwen move back into the neighborhood, taking over the house next door to the one where Max lives with his grandparents, the two boys hit it off as friends, and Max invites Kevin over to his house to see his room. When Gwen calls for Kevin to come home, he emerges from Max's basement room with Max; Kevin is afflicted by a congenital disease which results in dwarfism, and he looks very tiny and helpless next to his hulking companion. Gwen expresses her fear for Kevin's safety by hustling him quickly back to their own home.
It is true that Max's size in relation to Kevin's makes Gwen nervous, but Gwen is also bothered by the resemblance she immediately recognizes between Max and his father. Gwen had been good friends with Max's mother back when the boys were preschoolers, but Max's mother had been strangled by Max's father, who had been in a rage. Max's father has been sent to prison for life as a result of the murder, but seeing the son who looks so much like him brings back horrible memories for Gwen. At first glance, it might have appeared to Gwen that Max, the person who was with her son, was in fact his father, and even once she realizes that he is a different person, somewhere in the back of her mind, there is no doubt that she fears that Max might take after his father in more ways than their physical appearances. If Max shares the same violent tendencies as his father, then Gwen would behis right in fearing for her son's safety. Fortunately, Max is nothing at all like his father in temperament, and Max and Kevin become fast friends. After overcoming her initial misgivings, Gwen recognizes that Max is a good person who does not share his father's violent nature, and accepts him for who he is.
One of the reasons that Max makes the beautiful Gwen nervous in the book "Freak the Mighty" is because he is so big. Her son, Kevin, is very small for his age and has a contorted body. He does not really have any friends and suddenly there is this great big guy who is larger than other children his age who is hanging around with her son. Being the good mother that she is, Gwen is concerned that he may be a bully or not be right for a friend for Kevin.
However, it does not take long before Kevin becomes the extension of Max's mind and Max becomes Kevin's legs. Both boys have something positive to offer one another which results in a very special friendship.
That's a very easy one! There is only one answer. Max was VERY BIG!