Your dentist must have removed the infected carious lesion which has bacteria in it. When we do composite filling, the dentist just has to remove the carious infected part and there is no need to give the proper cavity form as earlier done with silver fillings because in silver fillings the retention is from the cavity formed which has to be prepared accordingly but in composite fillings the retention of the filling is by the bonding of material with cavity wall and due to this excess tooth cutting is not required. These days dentist go for conservative dentistry in which minimal tooth cutting is done so that maximum tooth structure can be saved. For more info on dental filling you can read these artlcles....  http://www.identalhub.com/articlecategory-fillings-19.aspx