Market cap and revenues, top performers .

Here is a list of the global top in Market cap (top 4 - future potential) and annual revenues (top 3 - present reality) .. 

What distinguishes them ?

Corp.    Market cap $b.     Revenue $b

                  (Sept '18).               2017

Apple.            1,100.                  229

Amazon.        1,000.                   178

Microsoft.      855.                      90

Alphabet.       840.                     111

.......

.......

Walmart          281.                    500

Exxon Mobil    352.                    244

Berkshire.       530.                    242

Hathaway ..

Why this anomaly in the global corporate scene ? 

The first thing that comes to our sight is -

a. the top four companies are all young organisations and from the digital domain
b. the largest corporates in the world in terms of sales are from the retail, energy and an MNC conglomerate which is many fields from hospitality, bank, transportation etc..
c. the large market cap of the digital companies, is it a phenomenon that will last for long is it just speculation given the novelty in the area ?

Has it got anything to do with the sector, future growth potential, technology, people, markets, leadership etc .. etc ..

George