Decomposition produces a variety of gases that increase buoyancy.
I have very high density fat. Although I’m more than 100 pounds overweight, I still sink like a rock when I get into the pool and exhale. If I inhale a lot of air into my lungs, I can usually keep my nose and mouth barely above water, but not much more.
In contrast, my wife is also overweight, but her fat is low density. There is nothing I have found that is capable of making her sink.
The times we have gone snorkeling, I have no problems diving and I have to work to surface. But she can’t dive for love or money, although she can get her face fully under water.