Stratovolcanoes can become larger than cinder cone volcanoes because

This is because they are built with lots of lava flow

The lava cools rapidly and builds up around the vent, forming a crater at the summit ... Stratovolcanoes are also called composite volcanoes because they are built of layers of alternating lava flow, ash and blocks of unmelted stone. They are larger than cinder cones, rising up to 8,000 feet (2,438 meters)