Sam Weir was originally meant to be tall for his age (as Feig had been).
Had there been a second season, Paul Feig wanted to have Sam breaking with Bill and Neal to find a new group of friends, Lindsay struggling with drug addiction, Kim Kelly getting pregnant, Bill's mom marrying Coach Fredricks and Neal's parents divorcing. And Sam would have gotten tall; John Francis Daley grew almost a foot in the year after cancellation.
Feig also wanted Lindsay and Daniel to eventually end up together.
On the topic of how Sam was supposed to be tall, Daniel was supposed to be Hispanic, and Ken was supposed to have long blonde hair. Neal was supposed to be named Art Thompson, Lindsey was supposed to be 15, etc.