As companies grow bigger, the de-modularization of the economy increases. Not something to be very excited about.
Why not? The economy demodularizes if and only if it is economically cheaper and more efficient to do so.
In other words, demodularization only happens when many, many people can derive a huge economic benefit from it happening.
The alternative is slower, more expensive, less efficient economic systems, filled with rent-seekers at every level.
By setting up their own distribution networks they control the entire chain from first click to final delivery, and to Amazon that is equally important as cost.
It also allows them innovate in an area that is sorely lacking innovation, all under their control.
This is the lesson of the new economy. You have to control all contact points with the customer. If you don't, you are letting a third party "sell" your brand which never works in your interest.
If I were to guess, I'd say that amelius is saying that it becomes increasingly common as companies grow for them to expand into functions that were originally outside their core function. Framed this way, "selling stuff online" may have once set an effective upper bound on what Amazon does, but now they appear to be expanding into "moving stuff from place to place."
This concept is related to vertical integration.