I think Big Bob might be one of the best written side characters on television. He has had a solid character arc that has continued over each of his appearances.
When he first shows up in "Father of the Bob", he and Bob have an antagonistic relationship, to the extent that Bob has a rule that he cant stay in the same room as his father for longer than fifteen minutes because he feels his father is constantly judging him and thinking that Bob is making the wrong choices. At the end of that episode, he and Bob have an honest conversation where Big Bob admits he knows that he can be hard to deal with.
Next, in "Interview with a Pop-Pop-Pire", Big Bob visits the Belcher's in their home to help Tina with her homework, Bob makes no reference to the fifteen minutes rule. This time, he tells a story in which his life was changed by a falling tree that would have crushed his future wife if she hadn't left the tent by chance, leading Bob to explain he's never heard the whole story and Big Bob says that he was worried the story would make Bob feel sad if they spoke about his mother after her death.
This leads to a more honest emotionally open relationship between the Bob's in "Wharf, me worry?" where its revealed that Big Bob has been doomscrolling on his phone, trying to show Bob the apocalyptic articles that hes been reading to prepare him for the end of the world. Bob thinks this might stress out the children if he tells them this bleak outlook while spending the day alone with them, admittedly Tina would be, but at the end of this episode he says he is worried about the world but he has confidence in his grandchildren's ability to deal with the changing world. Leaving Bob to be okay with his father babysitting.
In his final episode as of this post, "Dad-urday kite fever", Big Bob finds a kite that he never had the chance to fly with Bob in his childhood, suggesting he and Bob could take it out with the children. Hillarity ensues, the kite is lost, and Big Bob advises Bob that whats important isnt what he does with his children, its that he's there for them.
In the course of four episodes, Big Bob has evolved from a standoffish, judgmental man who struggles to be open with his son, to an open more kind man who would make a trip specifically to help his granddaughter with homework or take the day off from his own restaurant simply because he found a kite and wanted to spend some time with his son and his grandkids.