Big Baby, along with Lotso and Chuckles, was once owned by a young girl named Daisy, living at her house, as was revealed to Woody by Chuckles himself. Among them all, Lotso was the most special to her. One day, during a break at a rest stop, Daisy was playing with her toys when she stopped to have some lunch, after which she fell asleep and forgot them there.
The three waited day in and day out for Daisy to return to them, but on one rainy day, they decided to make the journey back to her house when she didn't come back for them; however, upon arriving, Lotso glimpsed through the window at Daisy playing with an identical bear. Angered that she had replaced him, he then convinced Big Baby that they all had been replaced and that Daisy no longer loved him, as well as intimidated Chuckles into remaining silent as to the truth.
The three toys then hitched a ride on the back of a truck before being thrown off by a bump in the road and landing in front of Sunnyside Daycare, which Lotso proceeded to take over using Big Baby as his enforcer, due to his considerable size in comparison to the other toys.
Toy Story 3
Big Baby is first introduced while Lotso is leading Andy's toys to the Caterpillar Room, where the bear assigns them to. Later that evening, after the toys experience a rather rough playtime with the toddlers, Buzz Lightyear emerges out of the Caterpillar Room and follows some of Lotso's henchmen into their secret hideout, the Vending Machine, where he eavesdrops on the gathered group, but is caught in the act by Big Baby, who throws him into sight of the group. That night, when Andy's toys are imprisoned in the Caterpillar Room, Big Baby separates Mr. Potato Head from the other toys by imprisoning him in a sandbox. The next night, Big Baby stands as a guard while Ken and a reset Buzz (who now serves as one of Lotso's helpers) take roll call to make sure that all the toys are there, then the three walk off to capture Mr. Potato Head after discovering a real potato inside his cell. Once again, Big Baby imprisons Mr. Potato Head in the sandbox outside after Mr. Potato Head kicks Ken in the shin, though this is revealed to all be part of the toys' escape plan.
During the escape, while Big Baby is rocking slowly on a swing and looking up at the night sky, Woody, Buzz (who has now been set to his Spanish mode by the toys by accident), and the other toys sneak past Big Baby as they make their way to the garbage chute (the only escape route that the Chatter Telephone has pinpointed to Woody), but they accidentally get Big Baby's attention when one of the Aliens trips, making it land with a squeak. Big Baby gets off the swing and walks over to the source of the sound, but the toys dive under a plastic sand bucket at the last moment, so that Big Baby finds nothing when he reaches the spot where the toys have been standing.
At the garbage chute, after Big Baby, Lotso, and the other Sunnyside toys surround Andy's toys at an open garbage dumpster, Woody brings up the subject of Daisy and shows Lotso and Big Baby an object he has received from Chuckles during his stay at Bonnie's house: Big Baby's old pendant. Upon catching sight of his old pendant, Big Baby suddenly remembers Daisy. As Woody tries to tell the truth about Daisy to Lotso (who doesn't want anything to do with Daisy) and Big Baby, Woody tosses the pendant to Big Baby, who picks it up to take a look at the token of her lost love and shows that he still cares about her, whom he affectionately refers to as "Mama!" Lotso suddenly insults Big Baby for loving Daisy again, yelling to him that she has never loved him as he snatches the pendant away from Big Baby, slams it onto the dumpster lid, and smashes it into pieces with his cane.
Seeing his last remaining memento of his old owner being destroyed, Big Baby begins crying, and then Lotso pokes Big Baby abusively with his cane for thinking like a dummy, causing Big Baby to yowl in pain. Realizing that Lotso has taken him away from Daisy and having had enough with all of Lotso's lies and bullying, Big Baby lifts up the vile bear from behind and throws him into the dumpster, then closes the dumpster's lid on Lotso, redeeming himself in the process. He then blows a raspberry at Lotso and helps Woody and his friends climb to safety.
During the credits, Big Baby is shown to be having a happier time at a now cool and groovy Sunnyside under the care of Barbie and Ken, the new leaders. At a disco party held in the Butterfly Room, Big Baby, wearing gold diapers (similar to Ken's tuxedo and trousers and Barbie's dress), is seen hugging Ken and Barbie in his arms and happily spinning the couple around in a spotlight.
Big Baby and his friends in a poster for Toy Story 3
Big Baby is usually silent, except when Woody throws a necklace that says "My heart belongs to Daisy" to Big Baby as he says "Mama?" while crying afterwards.
The scene where Big Baby throws Lotso into the dumpster mimics the scene in Star Wars: Return of the Jedi, where Darth Vader throws Emperor Palpatine into the reactor shaft of the Second Death Star.