Why aren't there as many gearboxes as file cabinets in EMR instances?

By default, EMR instances are configured with fewer gearboxes than cards. But reducers are not given additional memory, so it seems that they should have the same amount. (For example, super-large instances with a high processor speed have 7 cards, but only 2 reducers, but both converters and reducers are configured with a memory capacity of 512 MB).

Does anyone know why this is so, and is there a way to indicate how to use as many gearboxes as file cabinets?

EDIT: I had the wrong amount, it's 512 MB