I have a hard time understanding the comparison of aggregate market cap (a valuation) to GDP (an annual output). These seem like two very different quantities,

I have a hard time understanding the comparison of aggregate market cap (a valuation) to GDP (an annual output). These seem like two very different quantities, with different units. How do you intend for us to interpret that comparison? Wouldn't it make more sense to compare the aggregate revenues of those companies? (I'm not an economist, so I'm not sure those two quantities are comparable either, but they seem closer at least.)