A 99% confidence interval will be wider compared to a 95% confidence interval. This is because while you don't know what the true value of mu is, you are more confident that the parameter is in the interval.
In essence, you are casting a wider net which leads to the higher level of confidence. Imagine if there was a 100 mile stretch of straight land, and somewhere on this line was a special rock that you are looking for. You would be 100% confident if you said "the rock is somewhere on that piece of land", and less confident the more you shrunk the interval.
