The film was, at first, not supposed to be a sequel to Honey, I Shrunk the Kids (1989). Originally titled "Big Baby," it was about a young toddler who grew to giant size by a freak accident involving a growth ray and eventually terrorized Las Vegas in a non-violent, yet Godzillaesque way. Disney saw the possibilities of making this into a follow up to Honey, I Shrunk the Kids and rewrote the script to the movie. Whereas most of the characters from Big Baby were rewritten as characters from Honey, I Shrunk the Kids, there was no character that could take the place of Amy Szalinski, Wayne and Diane's eldest child and only daughter, portrayed by Amy O'Neill. Instead of excluding her character from the story, Amy makes a brief appearance in the beginning of the film, and it is explained that she is leaving for college.
Much of the the dialogue between Wayne and Adam, such as the bedtime story and feeding time, was improvised by Rick Moranis in response to whatever Daniel Shalikar and Joshua Shalikar, the twins who played Adam, happened to say.
Adam Szalinski is played by twin brothers Joshua and Daniel Shalikar. Most films and TV shows that involve small children typically cast older children (typically ages 3 and 4) that look younger than they really are, however the Shalikars were 2 years old during the film's production, the same age as the character they were playing was supposed to be, due to the fact that they were fairly easy to work with and took direction well. Dan was noted as being the more adventurous of the two, while Josh was more cautious.
Actor Alex Daniels portrayed Adam in his "blown-up" form (he is credited as "Uncle Yanosh"). Daniels wore a 40-pound, electronic-headed "Adam suit" for the role, and was coached on how to mimic the movements of a toddler. Once suited up on the set, Daniels had to magnify his movements so they would show through the costume's heavy, clumsy folds. Occasionally, the heat inside the outfit proved too much for its coolant system, a vest with ice water pumped through tubes, prompting crew members who noticed Daniels faltering to yell, 'Get Alex outta there!'
As a result of the film, Disney later found itself the subject of a lawsuit. The suit was filed in 1991 by Mark Goodson Productions director Paul Alter, who claimed to have come up with the idea of an oversized toddler after babysitting his granddaughter and watching her topple over building blocks. He wrote a screenplay titled "Now, That's a Baby!", which had not been made into a film but had received some sort of treatment beforehand. Alter claimed there were several similarities between the movie and his script, which consisted of the baby daughter of two scientists fall victim to a genetic experiment gone wrong instead of an enlarging ray. The case went to trial in 1993, with the jury finding in Alter's favor. Disney was forced to pay $300,000 in damages as a result.
Like Kate and Ruth Greenfield, the twins whom played Elora Danan in "Willow", Daniel and Joshua Shalikar did not act in any other movies. In real life, Daniel became a firefighter and Joshua became a Digital Technologist.
During the scene in which Hendrickson is getting the helicopter ready a US marshal walks up and hands a piece of paper to the pilot and walks away without dialogue. The extra who played the marshal is Dorian Gregory who would go on to play inspector Darryl Morris in the first seven seasons of Charmed.
When Dr. Hendrickson makes a second attempt to shoot Adam with a tranquilizer, it hits the Hard Rock Café guitar giving Adam a painful electrocution shock. The electrocution shock could had given Adam nerve pain and could had killed him.
The trivia item below may give away important plot points.
It's possible, when Dr. Hendrickson (John Shea) attempted again to shoot Adam (Daniel and Joshua Shalikar) with a tranquilizer, which hits the Hard Rock Café guitar, giving Adam a electric shock, causing him to drop it may not had been accidental and that Dr. Hendrickson may had shot the Hard Rock Café guitar deliberately, in order to force Adam to drop it, so he can get a clear shot at Adam.