On the DB-12, this is 250mm, which puts the effective cone area (Sd) at 490cm².
Sd is an important specification because it means the Velodyne DB-12’s cone can ‘drive’ nearly half a square metre of air! By way of contrast, a typical 254mm (10-inch) diameter cone has a cone area of around 230cm², which means that two DB-10s won’t move as much air as a single DB-12. So sometimes, size is more important than quantity!
* half a square meter is 5000cm² (50 * 100) and not 500cm²!
* a typical 10 inch speaker actually has a Sd of around 350cm². Maybe you were thinking of 8 inch woofers, which still have an Sd of around 250cm². As you go up in diameter, the surface ratio between consecutive sizes gradually decreases, so it's pretty wild to say 1 x 12 > 2 x 10
* the amount of air a speaker can move is calculated as Sd * Xmax and is a volume, not a surface. Subwoofers with a lower diameter can definitely move as much, or even more air than higher diameter woofers, if they have a higher Xmax to compensate. Seeing the size of the magnet and cheap construction on that woofer, it doesn't seem to offer anything spectacular in excursion (Xmax).