Check yu out, put on a couple of mm and your talking about stretching and shaving. We'll have you on stage in your doris's shit nicks oiled up with marmite yet!

Get some bio oil or similar on the chest/shoulder area. Very prone to stretchmarks there.

As for the growth, maybe try some flies to help the inner chest. TBH its probably down to genetics as the bulk of anyones chest is on the outside. Just look at someone of size doing a bicep pose and you will see most of their chest up by their armpit.