Chains like McDonald's do in fact waste large quantities of food (in absolute terms). I think this is largely caused by the even larger quantities they sell. But to a certain degree it is down to completely artificial and arguably pointless metrics:

1. They produce (or used to, this practice has been removed in Germany, I think) a large amount of food on stock and kept it warm. If it isn't sold after a few minutes (15?) it gets trashed. This is to optimize the speed of sales.

2. They hand out bizarrely large quantities (especially soft drinks, but also cheap stuff like fries) that often the customer won't eat completely. This is for a reason that I don't understand. Might be a US thing, though.