The main characters in these games are brave tiny heroes.
Most of the time, the hero of a story has to face a big problem to get to their goal. It can be hard for a normal-sized hero to travel through a world and deal with the problems that come up. But when the hero in question is shrunk down from their normal size, it adds a whole new level of challenge.
Many video games have some kind of shrinking feature, either because the hero starts out small or because they learn how to get smaller. But even though being small might seem like a bad thing, there are many good things about it. And there are lots of games where the heroes are small that show this in motion.