I think it's easier to make this kind of statement about how they measure up in terms of public e-commerce companies and market cap. Amazon's bigger. Alibaba's bigger. Who am I missing? I think that would put them at #3 in that particular segment.

Fun fact from some years ago: It used to be a head-to-head race for 2nd between Booking and Ebay (when ebay was at their peak and included PayPal).