Where is weight stored in the database?
Weight is an attribute in Magento's EAV system.
Look at the table eav_attribute. Find the row with attribute code 'weight' and entity_type_id 4. (Entity type 4 means products.) In my table, this is row 64. This means the weight attribute is attribute 64.
Now look at catalog_product_entity_decimal. This is where all decimal attributes for products are stored, and weight is a decimal attribute. All the rows having attribute_id 64 are weight values. The entity_id values correspond to the products.