Ape Shaman ape companion (8th level druid) has feats:
(2HD) armor proficiency (light)
(3HD) armor proficiency (Medium )
When giving him a "Protective item" of Breastplate armor, the ACP (Armor Check Penalty) of -4 gets added to the BAB,
Also the size of the armor is shown as medium instead of large.
Anonymous
Cannot reproduce. And 'medium' at the armor display is not the size but the type of the ermor