Why is it okay for gastric byp patients to eat less because they made their stomach smaller?

What if i don't get a gastric byp surgery but still eat what the patients eat on the daily..?? like tiny amounts of food? I'll still lose the weight? I'm just wondering how their bodies are nourished if they are consuming such little calories??? Isn't this what anorexics do? but is wrong for them?? the only difference is they didn't get their stomachs made smaller...