The way they got so big wasn't that they were super efficient
Joke used to be that they lost money on every sale but made up for it in volume. Turns out it's easy to beat competitors if you're not constrained by needing to make money.
Anyhow, it's been interesting to see Amazon's repeated forays into groceries in the UK. I remember the first launch with great fanfare and press releases about how amazingly efficient they were with their cunning algorithms and amazingly logistics etc etc an they the flamed out very hard after slamming into massive existing logistics networks already locked into brutal competition with each other.
Turns out Sainsbury's had been doing online deliveries since 1995, Tesco since 1997 and they were by no means the only entrants (e.g. Ocado from 2000 determined to do it all with robots). I think Amazon is now on its 4th attempt to get a foothold, and they've recently announced closure of all their stores, and delivery via their "partners" which are all well established supermarket chains.
If we had proper antitrust law enforcement someone would have noticed ages ago that Amazon was going around buying up competitors and shut that down but well, we don't.
Not just that: they can do other things like selling at a loss to put a competitor out of business, and make shitty clones of products which they will promote while suppressing the competing products.