It becomes tricky because what does 'factory' mean? Especially once it's scaled to the size of a small town, you start to see town-like buildings popping up. There's some stuff which is arguably necessary for the factory itself, such as the train station and logistics facilities, a test track, and things which are necessary for the employees directly such as employee car parks and restaurants, but then there are other things like a visitor centres, shops, gardens, monuments, museums, a Ritz-Carlton hotel. Also, are power and water facilities part of the factory? Even if that means that we're counting a reservoir as 'part of the factory'[0]?

How would you come up with an exhaustive list of which buildings contribute to the size of the factory, and which should be excluded?

Otherwise, you're just looking at the land owned or leased by the company, which is obviously a valid measure somehow, but it's hard to say whether it would lead to fair comparisons. I don't have the answers, just throwing it out for debate.

[0]: https://maps.app.goo.gl/wYLiYdHFioyR9Y8b6