Fallout and Fallout 2
The Vault Dweller and/or Chosen One receives a +1 bonus to their Agility, but their Carry Weight is equal to 25 + (15 lbs. x Strength). The extra stat point gained by this trait can be set on any other SPECIAL stat, such as with Bruiser and Gifted.
When combined with power armor, Small Frame can have some odd effects. The Vault Dweller's and/or Chosen One's intrinsic Carry Weight will be calculated using the 15 lb/point stat, while the "bonus" Carry Weight from the armor's Strength boost will be calculated with the standard 25 lb/point. This means that if the Vault Dweller and/or Chosen One takes the opportunity to improve their intrinsic Strength, their Carry Weight will decrease by 10 lb.
Fallout: New Vegas
If this trait is removed through the Sink Auto-Doc, any acquired Perks that have an Agility requirement that is no longer met will not be lost. For example, dropping from 5 Agility with the trait to 4 without will not remove the Rapid Reload perk if it was already taken.