All this stuff about how Apple is "as big as" or "almost as big as" or even "bigger than" Exxon--maybe I haven't read widely enough, but every one of those stories I've seen talks in terms of "market cap"--share price times number of shares outstanding.
Well now. A moment's reflection will tell you there is no reason on God's green earth to expect that all shares will sell at the same unit price as a single share. Might be less, often more. Indeed if it were to be the same, you'd have to count it as a crashing coincidence.
But of more direct interest--market cap talks only of equity, says nothing of debt. The whole enterprise value counts both. So, how do Exxon and Apple compare in terms of debt? There is a difference, although it's perhaps not as great as you might guess.
Start with Exxon. From the balance sheet, we see that non-current liabillities total about $93 billion--a bit over a quarter of the much-noted market cap. Of course this is accounting-speak: non-current liabilities include, e.g., deferred charges and the dreaded "other." Pure "accounting" longterm debt is $12.2 billion--pretty slim when you consider the size of the company, but still more than a rounding error.
For Apple, the comparable balance-sheet non-current-liability number is $6.67 billion--a trifle between friends. Apple's "pure" long-term debt number is just what you'd guess for a Silicon Valley hottie--zero, nada, bupkas, zilch.
So however you count it, once you throw in liabilities, the total Exxon value is still a good ways north of Apple.