There can be several reasons for a larger-than-expected stomach size at 28 weeks of pregnancy, as indicated by the scan results. One possibility is that you may have miscalculated your due date or ovulated earlier than expected. Another reason could be excessive amniotic fluid levels (polyhydramnios), which can cause the abdomen to appear larger. Additionally, multiple pregnancies, such as twins or triplets, can also result in a bigger stomach size. It's important to consult with your healthcare provider to determine the exact cause and ensure the well-being of both you and your baby. They may recommend further tests or ultrasounds to assess the situation accurately and provide appropriate guidance throughout your pregnancy journey.