That's not how stormwater management works. Land elevation, adjacent land, the profile of the land (how much groundwater is there?) on which the structure is built (and probably even more that I'm not considering) all affect how big of a retention/detention pond is needed. And these requirements change over time when it is discovered that older sites' management techniques failed to adequately manage stormwater. The Walmart site was built at least two decades ago, no? That's enough time for stormwater management guidance and policy to change.

Even then, it is improper to assume anything about the stormwater management needs of one site based on another unrelated site. But even then, Walmart's site seems to be completely surrounded by stormwater management. Even the northwest corner of the site is a detention basin. [0]

The most obvious and logical conclusion to be made here is that an engineer told Microsoft they needed to have stormwater management of that size, so that's what they put in their plans. No sweetness, just lawsuit prevention.

If they want to be sweet, they should be building huge nature preserves (and they have enough money to afford it) into these plans instead of trying to be greedy by building the largest possible structures (they think) they can get away with.

[0]: https://beacon.schneidercorp.com/Application.aspx?App=DodgeC...