What Causes Huge Blood Clot Discharge From Vagina After A Miscarriage?

I am currently going through my first miscarraige and im getting huge lemon sized clots..ive been bleeding roughly three weeks. Would have beem 12 weeks pregnant yesterday. Doc said scan showed preg ended at 5 weeks and empty sac is 2cm big. If the sac had stopped growing at that size, wouldnt that mean p pk axenta aite is small, and if so, where are these huge clotd coming from if theyre too big to come from the placenta site?