Jacob wasn’t quite as young as poor Sam was when he shrank. He was 14 years old and already on his way to being tall, even if he wasn’t quite as tall as the then-20-year-old Sam that found him. Sam decided to take care of the kid right away, even giving up his dollhouse bed without even being asked. He knew how much it helped him with coming to terms with his new size, and he wanted to help Jacob with that, too (I know, they’re all such sweeties and really it’s deadly cute). At the time of Brothers Adopted, it’s been 3 years since Jacob was cursed.
For a reference, here’s about what Jacob looked like back then (only with brown eyes):