Costco isn't for everyone.  But your 5 liter ketchup bottle example shows that you really haven't even spent 5 minutes in a Costco.

Sure you can find oddities like the huge cans of ketchup or tuna intended for commercial use - but you also can get a "regular" sized bottles or cans that might be sold in quantities of 2 for ketchup or 10 for tuna.  The regular Costco price is typically equal to less than a big sale price found at your local grocery store. 

Bread is another perfect example of how modest bulk buying pays off:  - for the brand we buy our local grocery sells it for around $5 per loaf regular price.  Costco regular price is 3 loaves for $8.

And yes even Costco often puts these products on sale resulting in even greater savings.

Not bad for having to flash a piece of plastic at the entrance and show your receipt at the exit.