Do you know what this bub is? Maybe its a boy who takes after his daddy
My DH and I were roughly the same size at birth (him, nearly 8lb and me just over 8lb) and although our bubs were both born early, they would've both been roughly 8lb at fullterm if they'd continued. So maybe, the 'taking after parents' scenario is possible.
Oh and one of my pregnancies was very stressful while the other wasn't and I took less care of myself the second time around too - so those things don't seem to factor
You're doing fine - enjoy the last of this pregnancy, hope to hear of your healthy bubba's arrival in coming weeks.