I don't usually "let" a faction get huge, but I often ignore factions to focus my efforts on gaining more profitable provinces, punishing backstabbers, or seeking other important provinces. As the Turks I generally don't ignore anyone but Russia though, I usually try to envelop the Med and maybe eventually take down Russia, other than them the English are usually the last to fall. It is just much easier to control trade, and the seas when you are the only one with a province on the Med and have 3-4 ships in the Straits of Gibraltar....

When I play as a Western European faction, or Central European faction I generally ignore the Byzantines, or whoever takes Constantinople.