I'm not going to write an (overly) long comment on it as I think Amazon is in a good position for continued growth but not because market cap is bigger (revenue is still smaller, employees is still smaller, footprint is still smaller, groceries have been a slow burner even post Whole Foods) and certainly not because numbers went up for the last 5 years so therefore they'll just keep going up until all competition is buried.

Also remember when looking at valuations/revenue/profits/growth that these mega companies can be extremely wide. What looks like massively higher profit margins turns out to be from Amazon Web Services driving 2/3 the operating profit not from store sales margins and even if it was that one company can sweep up a certain category of sales doesn't mean it'll be able to sweep all of them up.

There's also nothing to say Amazon can't be outdone by others or can't become the source of its own demise just because it's done well over the last 5 years.

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Anyways back to the XKCD question Amazon has experienced high triple digit growth in the last 5 years and still hasn't approached Walmart in sales/revenue. That doesn't mean it won't do so but that also doesn't mean the next 5 years will be as easy to have triple digit growth, especially if that growth was in enabling new sales rather than displacing traditional sales. Similarly the market isn't worried about Walmart fizzling out any time soon like Yahoo did, to go back to traded value despite Amazon's meteoric rise Walmart is still valued twice as much over the last 5 years compared to the 15 prior where it sat completely flat even though Amazon wasn't much of a competitor at all during that time.

Market cap is an expression of how much investors think a company's stock is worth.

For an apples-to-apples comparison of real growth, you want to compare _revenue_, not market cap. Amazon has not yet passed Walmart in retail revenue (as of 2020) though it's true their revenue is growing much faster. Hence the XKCD references.