Jacob was only 14 when he was cursed. Older than Sam was when he was cursed, but still just a kid. At the time of the story, he’s 17, so he’s still pretty new to the curse while Sam is a veteran. That means Jacob is stuck between worlds! He still remembers pretty well what his life was like before shrinking down, but he’s also getting used to the size of everything around him.
What a time to meet a giant Dean Winchester, huh?