I'm overweight not majorly but could stand to lose a few pounds and people where asking me when I was due when I was 15 weeks. It depends on so many things like position of the baby/placenta. If you've had a baby before then your muscles are already stretched and therefor you will show sooner.
The more overweight you are, the longer/harder it can be to see a bump.
There have been heavy women who couldn't even tell they were pregnant.