Early in the film it was established that Adam only had to be near an electrical power source in order to cause a growth spurt. When he was on the Las Vegas Strip, both Wayne and Dr. Stirling state he has to touch the lights in order to grow.
(at around 33 mins) When Adam is in the dining room, he knocks one of the light shades from a fixture hanging from the ceiling, but in the next shot, the shade is back in place.
(at around 10 mins) Early in the film Dr. Henderson storms out of the lab and walks past a portrait of Clifford Stirling. In the following camera shots, he is directly in front of the portrait then beyond it again.
Adam places what he thinks is a toy car on the foot of the Vegas Vickie neon sign, which slowly begins to collapse under the weight of the car. When he finally removes it after much imploring from Nick and Mandy, Vickie's leg springs upright into it's usual position. The mechanism that once made Vickie's leg kick broke down some six months after she was originally built, and was never repaired, her leg would not have uprighted itself after the weight of the car was removed.
Increasing the amount of space between molecules would do nothing to affect on object's mass. Diane could have picked Adam up, whether she was blown up or not. Likewise Adam wouldn't have quite the strength to lift a car.
Except picking up a 112-foot toddler would still have been extremely difficult and awkward for a five-foot whatever woman, and she needed to get Adam to hold still long enough for Wayne to shrink them back to normal size.
Wayne says that Adam has increased mass at one point, so the weights do make sense, but it is not explained where this mass comes from, nor how Adam's physiology can move fluidly without collapsing under its own weight with all of the added mass.
Except picking up a 112-foot toddler would still have been extremely difficult and awkward for a five-foot whatever woman, and she needed to get Adam to hold still long enough for Wayne to shrink them back to normal size.
Wayne says that Adam has increased mass at one point, so the weights do make sense, but it is not explained where this mass comes from, nor how Adam's physiology can move fluidly without collapsing under its own weight with all of the added mass.
Increasing the space between atoms of a mass doesn't result in simply a bigger thing. The mass would become vapor.
Wayne reveals that things and people must be still for twelve seconds in order for them to shrink, but in the other two installments, the people that shrunk were not still at all. It is explained in the dialogue that due to Adam's large size, his increased mass needs a longer period of exposure.
(at around 29 mins) Wayne tells Nick about the reason why he made them all promise to never tell anyone that him and his sister (which would have included the neighbors children) were once shrunk and lost to them (which happened in the original.) When Wayne brings that up to Nick, he claims that they were "shrunk and lost for two days", when in fact, that only happened for one day.
The kids spent a good part of the day they were shrunk getting back to the Szalinski house, and a small part of the next morning continuing the journey, having to stop to sleep overnight. This is similar to stopping overnight at a hotel, and as it was sunlit on two separate occasions with an intervening night, this would generally be regarded as two days.
Given how much electricity is used in the Las Vegas strip, plus Adam grabbing the electronic ice cream, and giant guitar, he more than likely would have become much bigger than what we see as his biggest size.
Around 01:05:28, we can see that the car is a plastic toy : color is now light brown and nobody is inside.
(at around 35 mins) When Adam is shaking Nick around: the camera changes angles and shows Adam holding an obvious doll made to look like Nick.
When the Hard Rock Cafe guitar crashes, it is clearly shown to be made out of sugar glass, and containing low voltage electric lighting.
At around 6 mins, the baby first drops an electric guitar face-down on the carpet then lowers it, the guitar sounds are overdubs and inaccurate. The first would mute the guitar, not cause it to chime loudly, and the second would have very little impact on the sound of the vibrating strings - certainly not a 'dive-bomb' effect heard which would usually be achieved with a 'whammy bar' which this guitar doesn't have.
Right before Adam goes for the guitar in front of the Hardrock Cafe/Hotel he can be seen in front of the volcano at the Mirage Casino. The Mirage is also on the Strip and not Downtown.
(at around 1h 2 mins) After Adam leaves the house, the US Marshal indicates he is headed towards the Las Vegas Strip when he was actually headed towards Fremont St. which is in downtown Las Vegas.
In one scene when Adam is on Fremont St. in downtown Las Vegas the marquee for Circus Circus is behind him. However, all the other signs are for casinos on Fremont St. The Circus Circus sign is not shown again.
The final scene--and the punchline--of the first film features the shrinking machine in reverse, and the family feasts on enlarged food. In fact, reversing the effects of the machine is how the kids went from tiny to normal size. In the second film, Wayne is working for a company that is testing a completely different machine that will enlarge things, and the shrink machine is in storage. It is never explained why simply reversing his shrink machine wasn't the answer.
At the end, Wayne and Diane seem to think that shrinking Big Bunny will be a problem, even though Wayne was able to shrink Diane and Adam back to normal size when they were both MUCH larger than the bunny.
(at around 54 mins) When Clifford asks Wayne what could be causing Adam to grow, Wayne says "Yesterday, when I gave him lunch, he was right near the microwave oven", when, in fact, that happened earlier that same day.
Wayne says that Adam needs to be still for 12 seconds in order to be shrunk. When Adam's stuffed animal in enlarged and he cuddles with it, he is still for longer than 12 seconds. They could have had him normal sized right then.
(at around 27 mins) When Dr. Hendrickson asks Wayne Szalinski what language Adam was speaking, Wayne responds, "Yugoslavian". There is no such language as Yugoslavian. Therefore it was a very decent cover-up for the baby talk Adam was naturally talking.