They don't have to be large purchases, just everyday routine stuff. For instance, going to work, somebody might get a bus and a train. That's two transactions per trip, four transactions per day, 20 transactions per week. 25 if they grab a coffee on the way to work. 30 if they buy lunch. You're looking at ~1,500 transactions per year just for one person with this routine, even if they don't use Apple Pay for anything else. There are about a billion active iPhones out there, it doesn't take a large proportion to start using Apple Pay more to account for a huge number of transactions.

As soon as a city rolls out NFC support for public transport, that's going to be an instant boost of millions of transactions per year, because anybody with a smartphone or smartwatch doesn't have to bother with the hassle of buying tickets or topping up a transit card any more, they just have to wave their watch at the reader. Compare a transit card, where people typically top it up for weeks at a time with a single cash or credit card transaction, to Apple Pay, where there's at least one transaction every single day you travel.

I'm used to living in places where NFC support in public transport is ubiquitous, and when I visited San Francisco recently, it felt like I was living in a time warp having to mess around with a Clipper card. BART has ~125 million trips per year. How many transactions per year do you think Apple Pay will be boosted by when BART finally supports it? And that's just one single location for one use case.