Scout is still the voice of a little girl when she says this. She compares Atticus with the status-quo of fathers in Maycomb. Most dads are much younger than Atticus so she derives that he is physically feeble. It isn't until Scout matures that she begins to realise how special her father is. As Scout becomes a stronger individual she begins to see the inherent strength in her father. In chapter 10, Scout sees her father shoot a rabid dog from far away. Ms. Maudie informs Scout that her dad is the best shot around: they call him "one shot Finch". Scout sees this as "strength" she once saw lacking in her father.