I don't know about McD's, but I work at another very large franchised quick-service restaurant (QSR) (not McD's, but in a different category).
For companies like these, that operate in 10's of thousands of franchisee-owned locations, the number of products and the combination of configurations is not merely a function of the things you see on the menu. It's all of those products, with their different combination of parts (beef patties, lettuce, etc.), and then franchisee and regional variations (In some countries, you can't tell a franchisee what they can or can't sell, etc.)
Add on top of that customer modifications to the product in their order (extra pickles, no onions, etc.)
Why does this matter? It drives lots of stuff - food costs, inventory, inventory & sales forecasting (how many pickles do I need this week?) new item research, profit margins, etc.
So you take this, multiply it by 10,000, 15,000, or, in McD's case, 36,000 stores across 100+ countries (half both those numbers in my company's case) and you're talking about vast amounts of information across millions of transactions every day, and that is in fact Big Data.
Whether this is a smart move, I'm not sure. I'm of the opinion that if a piece of technology is core to the _operation_ of your business (for a QSR, things like POS systems, backend administrative systems, etc.), it should be in your house. Other things (HR systems, big data systems) should not be. But maybe McD's sees it differently.