Just 'spitballing', but if the restraints are adequate, the compression of flesh may act to slow down and spread out the force of the accident to a certain degree. Bruising may be more likely, and the increased overall force might break ribs and such, but increased weight may have some benefits at some speeds/in some scenarios.

Some possible downsides would be overwhelming the airbags capacity, and larger overall volume of space which needs to be protected from intrusion/crushing/etc. I Wonder how much this has been studied, hard rubber and steel dummies probably don't reflect conditions at the extremes very well...