What is my due date?

First my ob told me that my due date was 12/25 according to first day of period/day of intercourse etc. I was very precise with this information since I kept track of everything. A few weeks later they measured me and the baby and said that my baby was rather small, so they pushed the due date back by 1 week to 1/1. I have been sick the first 6 months and lost 24 lbs. But now they are saying that my baby is bigger and heavier than average. So I feel like I should rather be due on my initial calculated due date. Anyone who's had experience with this? What was the outcome?