That sounds like it's basically the grocery equivalent of the boot theory of poverty. Poor people have to pay more in the long run because they can't afford to buy in bulk.
It’s slightly different for groceries. I am not poor but I also don’t want to buy perishables in bulk. I can choose to buy one week’s worth of lettuce to be eaten in a week, but by the seventh day the lettuce has visibly degraded. I want fresher produce, so I am willing to buy smaller amounts every two days.
When they say "groceries" they're not just referring to fresh produce but also to things like cereal, dried goods, canned goods, or other foods you might find at a dollar store. Though some of these stores like Dollar General do also stock fresh foods like eggs, meat, dairy, and produce.
This is exactly right and the reason that Costco shoppers are un-intuitively among the richest groups in the country (average $125,000 household income).
Costco is great for wealthy families, less so for less wealthy. People living in small apartments have no place to put 36 rolls of paper towels and 12 jars of pasta sauce.
Having a large home is a prerequisite for shopping at Costco.
We live in an apartment but use Costco to stock our freezer with meat and seafood. We also use it for gas, cat litter, eggs, and cheese (lasts a long time). Basically for perishables that only need to be stored so long.
In addition to the other comment about perishables, storage space is another meaningful limiting factor that can vary with income level. Both the raw volume of available storage and the quality of the storage on things like temperature control, energy usage, accessibility, etc
