This is a new addition of Pathfinder Please eliminate or untie the rule that you have to be proficient with light or medium armor to be Proficient with heavy armor. It make NO sense. Proficiency means trained in. You do not have to be trained in light armor to be trained in heavy armor.
all it would take is an arms master to put you in plate armor and train you only in that armor for you to be Proficient. Make each type of armor its own Proficiency. Make feats to make one better in each type of armor.
Why would a Paladin even bother learning Proficiency in light armor
at the most they would learn Proficiency in medium armor or Heavy armor.
IMO the only reason a Paladin would b trained in Medium armor is that the Temple was poor and did not have old suits of heavy armor to train their novices in. Paladins have many more important things to learn than the proper wearing and fighting in armor that they will only wear when they are Novices. Temples would have fundraisers to buy heavy armor for their Paladins or better yet trade healing with the armorers guild when their members get hurt. This same logic works the same way with fighters to a lesser extent. Fighters would train in schools or guilds and rent their members services to caravans to pay for armor for their members. enviormental conditions would be a more determining factor if a fighter was trained in light or medium armor than heavy armor.