Here is a visual example of what 5 lbs of fat to muscle looks like.
Fat will have more to store around the body at 5lbs vs. muscle. Remember, muscle weighs more than fat at a more dense level.
Muscle requires more energy, therefor you must feed the body the proper nutrients to sustain attainable levels of maintenance or growth.
Resting energy expenditure is going to be higher with greater lean mass.
If you take two people of equal weight, but one has a higher ratio of lean body mass to fat mass, he will burn more calories at rest than his counterpart.