That's even more true with small spendings: with cash, you easily see the amount stocked in your wallet decrease item after item, shop after shop, drink after drink; you also need to "refill" at the cash dispenser when it gets empty so that reminds you that you have already spent the XX or XXX € you had taken 2 days ago; you also may not be bothered to refill and opt for a lower priced item, or a lesser quantity of items, or do without the item.

If you start paying small amounts with a card, it is too easy and also you don't keep track of your spending rate, you don't "feel" it, so you end up spending more.

(After all, that's the point of it, shopkeepers wouldn't bother losing time and money to offer card payment if they got no benefit from it.)