In Williamson ether synthesis, the reaction begins when the hydrogen from the alcohol's hydroxyl group contacts the halogen ion. Since the iodide ion is larger than the chloride ion, there is a larger chance of collision between it and the hydrogen atom, so butyl iodide is preferred over butyl chloride.
Also, Williamson synthesis is carried out at relatively high temperatures, around 50ºC-100º C. Ethyl iodide has a boiling point of 72.2ºC, so it will be in the liquid phase. Ethyl chloride has a boiling point of 12.3ºC so it will be in the gaseous phase, which is undesirable for this.