“Then Abimelech said to Isaac, “Go away from us, for you are too powerful for us.” And Isaac departed from there and camped in the valley of Gerar, and settled there.”  Gen. 26:16-17

The Philistines envied Isaac (v. 14), and so this is not a reference to Isaac being a threat to Abimelech, but rather jealousy, and so the king is telling Isaac to move out of Gerar.  Isaac had become quite successful in his short time in Gerar and was living in abundance.  Ramban tells us this is likely because Abimelech was embarrassed by Isaac’s superior wealth.