Let's say iced tea costs the company 10 cents per L to produce.

• If you buy 500 mL for $2.80, the company makes $2.75 profit
• If you buy 1L for $3.00, the company makes $2.90 profit

So you feel like you're getting a good deal, and the company turns more of your money into their profit, even after accounting for having to make more product.

TLDR: Bulk pricing encourages consumers to buy more, which makes the company a bigger profit.

(Yes I know I'm ignoring packaging costs and stuff. frodeem's answer covers these economy-of-scale considerations very well)