The top DPS needs to be awfully far ahead of the pack for it to be worthwhile to extend just them rather than give the whole group.

Let's assume that all the DPS are equal. Assign them an arbitrary potency value of 100, over one minute of time. In a full party, then, after a minute you have a total of 400 potency.

Now, let's give one DPS an Extended Balance. Now, one DPS is doing 120 potency over that minute instead of 100, for a total of 420 potency.

Now, let's give all the DPS Expanded Balance. Now, each DPS is doing 55 potency for half that time, and 50 the rest. Again, that amounts to 420 potency, so it might seem like there's no difference - but don't forget the healers and tanks also benefit. It's not a whole lot more damage, but it's still more damage.

It should go without saying that Enhanced Balance is a waste. One DPS does 65 potency for half a minute, leading to a total of 415 potency over the full minute.

So, for it to be worthwhile to Extend one DPS instead of Expanding the whole party, that one DPS has to be doing enough extra damage over the other DPS to make up for the bonus to both tanks and both healers.