Shardblades weigh less than steel would of the same size, I see no reason Shardplate can't be the opposite.
Let's use the wineskin example that is used in the books, a human is like a wineskin with a hole in it, wine/light leaks out very quickly, the higher the oath level the less light leaks. However breathing is like opening the mouth of the wineskin and turning it upside down, no matter how small the hole in the bottom is, the mouth is the same size.
Alternatively, your mouth/nostrils are much bigger than your pores, they are the main way your body releases gas, just as carbon dioxide is released when you breathe light is too. However unlike with regualr gases, Humans are not built to hold stormlight, so it will leak through their skin. By saying oaths humans become more attuned to hold stormlight, so their skin leaks less, however their respirory system will still expell gas just the same.