A legend from an ArcMap service appears too small or too large when requesting it using an HTML or Custom viewer.
The autoextend property for the GET_LEGEND AXL request may be set equal to false.
Solution or Workaround
Check if autoextend = false in the request for legend is used.
Always use autoextend = true if the exact width and height for the legend is unknown. The width and height of the legend then defaults to the maximum width and height required for displaying the legend elements.