It's not about breasts. It's about domination and humiliation, common themes in fetishes.
Fat does not feel nice everywhere, boob fat feels nice when it's firm when the collagen and elastic fibers are strong, as they are in young women with perky breasts. Fat people are floppy and watery because their collagen and elastic fibers are inflamed, which gives a really disgusting icky feel to their skin and their fat.
So no, it's not about feeling good like breasts, it's the opposite. It's about humiliating the fat person or humiliating the person "enjoying" the fat person. It's about force-feeding, or being forced to feed. It's about being disgusting but still having someone love you. It's about all or any of the above.