So one way to do this is to create 2 labels, where one only shows if the %age is the largest, and one for the other values. We can then place these side by side and make the largest ones bigger...
So first I create the calculation for the Share
[Share of Total]
SUM([Sales]) / TOTAL(SUM([Sales]))
Then I create a Label calculation which only returns a value for the largest one...
[Share of Total - Max]
IF [Share of Total] = WINDOW_MAX([Share of Total]) THEN [Share of Total] END
with no ELSE condition...it returns NULL for all but the MAX one
I then create a Label for the rest
[Share of Total - Others]
IF [Share of Total] < WINDOW_MAX([Share of Total]) THEN [Share of Total] END
and then finally we can bring both onto the Label shelf and arrange the label (in Edit) like the below
and we get something like this
Hope that makes sense and is what you were after...let me know if not (on either front!)