Does a pregnancy sag still grow after miscarriage?

i went for a scan at 6 weeks and the doctor said everything was fine no concerns as i was spotting a bit. 2 weeks later i have had no bleeding or cramping and the scan showed a pregnancy sac with no baby inside. i had an internal and external examination. they told me it was a miscarriage yet the pregnancy sac had got alot bigger in size from the last scan at 6 weeks. they had advised surgery to remove it but im not 100% convinced the baby has died as why would my sac still be growing aswell as my belly? it seems early to be showing but it is definatly a pregnant bump and hard. this is my 2nd pregnancy and the 1st went fine with no problems.