Big products need to be put in a big container. A small container just won’t do. These products are big and they need a big container to be contained in.
Once you get the big products into a container, you then lock it with a key (き). You wouldn’t want these products to bust out of their containers, so it’s best to lock them up with a key.
Currently there are no similar looking kanji which exist in the WaniKani learning system