The key is uniformity of the weld. If you were to put a straight-edge against the bead, ideally the entire edge would be touching. That's why tighter beads are stronger, they're more uniform, the heat is the same, the penetration is the same and the shoulder (that mite not be the proper term) is uniform, no stronger or weaker points throughout the weld. Same reason trigger welding is bad, the cold start on a mig is far worse than the "throttled back" portion of a tig weld, but same basic concept.
Brian Mapes example is perfect (((((((( is better than CCCCCCC Each of those dips at the top and bottom of the C where 2 meet is a thin spot where weld strength is less than optimum.