Louis is well aware of the risks involved with him as a herbivore dating a carnivore like his beloved Juno. But he has found a way to curb any potential cravings she might have for his flesh (in any way that isn't sexual), and that is to keep her constantly full, ready to burst full.
The rational is that she won't go savage and eat him if she's too full to eat anything at all. Naturally, being fed so much food so often has made Juno gain a tremendous amount of weight. But because she loves Louis she is reluctantly going along with it as it seems to be the only way to keep Louis safe from any carnivorous urges that she would otherwise have an incredibly difficult time controlling.