I did interview with Microsoft Global Network Services several years ago (before Satya Nadella) and got a sense on the infrastructure and money they were pumping into Bing.

They were replicating the style of data center deployments that Google was known for in terms of scale and efficiency. I wasn't really privy to any specific details beyond the Clos architecture they'd adopted for their underlay and the fact that they rolled their own switches and SDN.

I don't recall specifically when this was, but I do recall being impressed that they had something deployed and working. The only folks anyone were certain had done this at the time was Google. Facebook would later announce something similar with their Wedge switch and now there's a fully baked OCP standard.

They point of all this I suppose is that they were heavily investing in scale for Bing and Office 365. I suspect the product of all that work is what became Azure.