Again, part of the overall drink is supposed to be made of the water from melted ice. It’s hard to compare sizes perfectly with that in mind, but it’s a less-extreme similarity to comparing a jug of orange juice with a can of concentrate (the kind you mix with water in a pitcher at home). Different products, but also different sizes for a reason.
I’ve never worked at a coffee shop, but I know our taste sensitivities are different at different temperatures. I’d be surprised if they were exactly identical products to account both for that, and the fact that your product is about to be diluted when it gets combined with ice (depending on the temperature before that happens). The ingredients may be more or less the same, but the proportions and concentrations may not be.