not sure which day my milk came in but my son was exclusively breastfed from the start and he weighed 9 lb 1 so don't worry about bigger babies being hungrier, you should be fine! the midwives were great, really helpful and tried to help me latch him on at every feed (didn't work that well for me - i was in agony for about 3 months but he was fine)
much better than first time where my son was on bottle by day 2!
good luck!