They probably think they’re saving money by buying larger packs. Which is true. But not if you have to keep throwing half of it out.

This is one of the main problems when we live sharing a house or flat. The different unrealistic habits will clash. It usually develops from annoying to angry very fast. I found one argument that might help is cost.

I had a similar problem, ended up throwing out a ton of food I never got to, what helped me was freezing everything I could (obviously mayo will get rekt in a freezer) and a friend who would add up how much I was wasting, like I would pay $9 for the XL jar and throw out 2/3 of it and that's 6 bucks in the trash when there is a smaller jar for $4.... in a few months I think I had wasted like 2-300 dollars, I wish I had kept the numbers but man it was too embarrassing

I’d probably do what inhahe suggested.

It’ll take up less space? You wont eventually have to eat 2 year old mayo?

if telling them that directly doesn't change their mind then there's probably no reasoning with them 🤣 i guess you could take a scientific/mathematical approach and figure out exactly how much money they'll likely lose by buying the large items instead of the ideal sized items..