In "The Oasis," Dr. Smith quickly grows to a giant size after becoming infected with alien bacteria that affect the pituitary gland. However, his clothes, shoes, and belt simply (and inexplicably) grow with him.
Becomes Fridge Horror when you realize that Will Robinson would have found him without any clothes on had they not grown with him.
Fridge Horror
"The Promised Planet" features an alien species whose members never age into adulthood. Instead, they remain as teenagers both physically and mentally for the duration of their lives. Now, the question of how they reproduce isn't really brought up, and the simplest explanation would be... not very comfortable to think about.
The Movie
Fridge Horror
As mentioned elsewhere, the movie ends with Smith starting to experience the effects of a spider bite, which we know will ultimately turn him into Spider Smith. The only one who knew how long it took for the changes to start (Future!Will) died when the planet tore itself apart , leaving them with a ticking time bomb among their crew, who already doesn't like the Robinsons and has every reason to hide any abnormalities (because he's reasonably certain he'd be killed). How long before he starts reenacting scenes from The Thing (1982)?
Alternatively you could reason that it took thirty years for Smith to mutate, having no medical treatment. In this new reality they have a warning of what could happen and time to cure the bite before it advances. He also has more time to reform.
Additionally this time Smith has seen himself as the mutant in the future, rather than growing into it. He doesn't seem eager to become that; he would encourage the hunt for a cure and he would have assistance.
Plus there's no way to be sure if other factors affected the mutation, such as prolonged time on that planet with exposure to something in that planet's atmosphere; whatever else happens to Smith, he won't be exposed to those exact same circumstances.
If all else fails, it serves as plot thread to deal with in any potential sequels. Not that this seems likely in hindsight...
