The red team have the large zorbs because there are fewer of them. They win if the Red Queen makes it past all obstructions to the edge of the board and
The black team are more numerous and have smaller zorbs so most times Descartes' hunchbacked minion Igor* would get roped in to play, along with the midget jester, the cook's kids, two ducks and an otter. They win if they capture the Red Queen (you capture an opposing zorb by sandwiching it between two of your own side, except the Queen who needs to be surrounded completely). To make it fairer (this is a big ask) they're allowed knight moves on the board as well as rook moves. The extra move is partly to accommodate the movements of Spatty the kitchen cat, who turns bolshie when she's shut in a zorb to make up the black team numbers, and cannot be trusted to follow Descartes' commands.
Originally Christina used to play Live Tafl using her household cavalry. She switched to zorbs because her advisor was scared of horses.**
Three games before breakfast was the limit because any more and one zorb or another would inevitably spring a leak. Christina bought them from a second-hand zorb yard. Would you trust someone calling himself "Honest Athanasius Kircher"?
* Pronounced "Eye-gor".
** It was a mistake to put Descartes before the horse.