No two appetites are exactly the same, and there are myriad reasons why a grown-up person may eat out at a restaurant and not want to order a full-size meal to eat. Depending on the place, some establishments are more than generous with their portion sizes, to the extent that even a hungry person may struggle to finish everything that they order. On top of that, various health and weight-related issues can make it worthwhile to choose a smaller meal than usual.
It makes perfect sense that a restaurant would want to cater to as wide a range of customers as possible, and part of that involves giving them an option when it comes to how much they want to eat. The most common smaller portion sizes come from kids' menus, and truth be told, many businesses could think about expanding their horizons when it comes to portion inclusivity. Ultimately, it is one of those things that has to be done at their discretion, and it is up to the clientele whether they accept that or choose to take their hunger elsewhere.