I'm not qualified to interpret results, but this paragraph stuck out to me:

> Using mice for the study, the researchers found that heart muscle also decreased in both obese and lean mice. The systemic effect observed in mice was then confirmed in cultured human heart cells.

So it also happened for already lean mice (though no mention of whether they still lost fat), and for cultured human heart cells (so not a by-product of needing less muscle to pump blood through a shrunken body).