I cant remember which account it is, but either Happycosleeper or Cosleepy did some posts recently about chest to chest sleeping that was really informative. E.g. There's some evidence that the puffs of CO2 from the parent encourages the babies breathing. I'd recommend looking at the post if you can find it.
I'm not sure how much evidence there is on this but supposedly our breathing helps regulate baby's breathing. Also, when we have baby sleeping on our chest we're sat at an incline so the baby's weight is distributed towards their bottom and not putting pressure on their lungs. Laying flat on their belly apparently puts weight on their lungs and reduces their oxygen levels. As I said, I don't know how much evidence there is backing that, however when parents started being told to put babies on their backs SIDS cases fell by 80%
Also intersted because our little boy has just turned 5 months and loves sleeping on his tummy - he rolls that way while sleeping and then sleeps really soundly. I'm constantly checking him so doesn't do wonders for my sleep but I found if I moved him onto his back again he wakes up and screams... just worried I should be moving him anyway because of this!