in absence of other complications, genetics is the main factor... also, your own height is: if you're taller, it's more likely that a big baby will have enough room to grow really big (while it may come a bit early if you're smaller size... that's a case in my family, all big babies, and many of them really huge even when coming early, without a single DG case! ...ie. i was born at 33 weeks and was 7lb 11, my cousin on my mom's side was 9lb 14oz (!!!!) at 36 weeks... my mom's tiny and my aunt is taller and when i arrived the docs said that if i would have had more space, i would have stayed in longer and gotten over 9lb easily)
sometimes, also the sex of the babies due to genetic imprinting counts (not that babies of one specific sex are bigger than the others GENERALLY, but within one same family, you can notice ie. that all girl babies are bigger than all boy babies or vice versa)... i am bracing myself to a little elephant.