SD cards do not increase the memory available to user applications. They are storage devices, even if the technology behind them has the word memory in the name.

It's not the memory that's the problem, it's that games like FGO require 10GB or more of flash storage which is most of a 16GB device particularly when you consider overhead for software updates. It's not like the game has high memory requirements but it sure has a lot of levels, textures and stuff.

It's amusing the the IBM Mainframe world still uses the word "storage" to describe RAM.