No matter what you say, do not mention negative things about your employer, neither should you mention anything about more money being the reason. The reason is simple; if you are leaving a company because of money to come to theirs, you will definitely leave them to move on to another if it promises a better paycheck. Your best bet is to ay it on responsibility and challenge and how your previous position want challenging you enough. Indicate that you yearn for more responsibility and how what you have to offer outweighs the responsibility and challenge posed by your former position.