Why would you think that a company's market cap (not only the relevant portion of the business, but the entire company) is a reasonable marker for how big of a player they are inside of this part of the industry?
Heck, market caps at this point are almost entirely untethered from reality. {cf. Tesla}
Market cap is a reasonable proxy measure for how much money those companies can bring to bear to win the market (especially if losses[1]), should those companies decide that competing is a number one priority. Two examples from Microsoft: XBox (worked) and Windows Phone (failed).
Revenues or profits in the cloud market for each company are mostly a measure of how much they are winning. How much they are spending is a measure of how much they are trying to compete, and the amount they can spend is also dependent on profits in other areas of their respective business.
> Heck, market caps at this point are almost entirely untethered from reality
Most stocks have some basis in reality, and relative value still matters even if you think the whole market is in Lala land. The stocks mentioned are not diamondhand stocks. Variation in valuation is not hitting two orders of magnitude, which is what we have here.
A better measure might be some gross profitability figure for each company that measures how much each company can pump into competing (expenses), but that is hard to calculate, especially for Amazon.
Heck, market caps at this point are almost entirely untethered from reality. {cf. Tesla}