What does it mean when my doctor said my baby was measuring large during my ultrasound?

Wide range of normal. The best person to answer is the doctor; a patient should never hesitate to call him or her and ask for clarification. The range of what we mfms consider normal for a baby of any age varies a lot. What is most important is determining the most accurate conception and due dates possible so that we can determine where the baby's estimated weight lies. A followup us would usually be helpful as well.
Different reasons. Your baby may be measuring for a number of reasons 1. Your last period may be unsure so your due date could be changed by the ultrasound 2. Your size may be off just based on the sonographer, or in the 1st trimester (1 week off), 2nd trimester ( 2 weeks off) etc.. 3. Or you may have gained a lot of weight this pregnancy 4. Or big people may big baby (ie your husband is 6 ft tall etc...).