Jar,
As Tim says, you've already done the damage with the mortise and making it longer doesn't have much effect. Also, making the tenon narrower is actually more work, since you will cut the shoulders first with the tenon full width and then cut it narrower if you need to. The only time you need to is to provide some relish if the mating mortise is at the end of its timber, or if you have an opposing tenon coming in that needs clearance. Thus, the rule of thumb is that tenons are the full width of the timber unless there's a good reason to do otherwise.