I'm not so sure about food, but for many mass market products it is indeed true that the same manufacturer can be engaged to make the same product under different branding.
The difference then comes down to the corners cut to meet the client's pricing.
Crappier boxes, thinner bags, packing material, and quality inspection. Assuming the core ingredients are not compromised in some way.
No you dont. I have worked in 2 groceries stores, the bags with less air get way more crushed and broken while stocking. Having bigger bags with a lot of air keeps the chips integrity in tact.
What is the company's incentive to make the package bigger than it needs to be?
Shipping costs come two fold... Weight and number of pallets. Weight change is negligible here, but the amount of air they need to ship will increase. They are incentivized to reduce it to a minimum to save on shelf, storage, and distribution costs.
They're also incentivized to keep the same size packaging (both for logistical and public perveption reasons) and ship less product in those packages. People are willing to pay $6 for a big bag of chips, despite the big bag weighing 150g less than the normal bag 5 years ago.
They don't get paid by the gram, they get paid by the bag. A bigger bag looks more impressive, and thus can be sold for more. Same for those tall skinny beverage cans. They look bigger than the regular cans, but are actually 25ml smaller, and yet go for a similar price.
This will continue until the price per gram is what people look for (emphasis on this at the point of sale would help), or the mass of each product is standardized. 50g, 100g, 200g, 350g, 500g, 750g, and whole kg sizes only, none of this 489g nonsense.
I don't agree with the can example. Those are physically smaller and lack meaningful slack fill.
Your points stand for the first purchase. After that people will know the proportion of chip to air, and be annoyed by it. If they could do a bag smaller with minimal chip breakage and less air they would both succeed at getting more bags out per pallet and be lauded for not cheating people by selling air.
The slack fill is functional, and I don't see much incentive to over do it.
You underestimate how little people think when purchasing things. None of this would be a problem if everyone looked at the price per 100g first, but ooo 3 $5... And then the size reduction usually goes alongside a packaging change, like jumbo or family size; "New look, same great taste!". It's all a distraction, out of sight, out of mind and all that.
Also, the 330ml cans are taller, and because of the square-cube law they only need to be a little skinnier to be smaller. They're also not usually displayed next to the normal 355ml cans. Out of sight...
Also, who is going to laude a big corp product for a logistics change in the first place? I barely see anyone complaining about shrinkflation for packaging reasons as it is. I'd see a better slack fill level on one product and think, "This must be old stock" or "This is the last time we'll get bags this dense".
The one example I'm familiar with is a name brand ice cream company that produces the store brand ice cream too...in that case the recipe is different, cheaper ingredients to cut costs to the bare minimum. But using the machines for a higher volume saves money.
I'm sure 'same exact item' does happen too but just 'same manufacturer' doesn't mean exactly the same item.
My sister worked at a dairy for a while, they both made the name brand version of cottage cheese as well as the off brand. They made several brands of cottage cheese, so you are abolutely right that different brands of product are made in tye same factory, but depending of the brand or country it was shipped to the recipie was changed slightly based on the customer's request.
Butter. I read somewhere sometime ago in a galaxy far far away that there is only a handful of US butter manufacturers which make all the butter for all the brands. Just different packaging. I have 0 proof or evidence and going entirely off memory of prolly a reddit post 10 years ago so google it and lmk if it's true.