Are the measurements on length? My ds was so long at birth: 55 cm but was only 3.5 kg which was the same weight as dd who was only 51cm. He had a smaller head too and it was a lovely birth. As arimeh says, tall kids might measure big inside but doesn't mean they are 'big' babies.

My midwife thinks your body grows the baby to the perfect size and then the eject button is activated! Both mine were 3.5kg and one at 38w next one at 40w. You might find you go into labour earlier than last time if he's big, but you have done it before, so I would put faith in your body.