Reduce in Stature

• WWizards of the Coast2015-02-25
  Official Ruling
  Effects that modify the power or toughness of the creature, such as the effects of Giant Growth or Honor of the Pure, will apply to it no matter when they started to take effect. The same is true for counters that change the creature’s power or toughness (such as +1/+1 counters) and effects that switch its power and toughness.
• WWizards of the Coast2015-02-25
  Official Ruling
  Reduce in Stature overrides all previous effects that set the creature’s power or toughness to specific values. However, effects that set its power or toughness to specific values that start to apply after Reduce in Stature becomes attached to the creature will override this effect.

Rulings via Scryfall