The result of an ArcMap Service legend request has empty white space at the top of the legend image. The amount of white space above the legend changes as the map scale of the service changes.
The ArcMap Service contains an ArcIMS Image service or a different ArcMap Service as one of the layers within the map document being served.
To workaround this issue, use static legend images in the viewer. Create and store legend images for the ArcMap Service in question. If the map contains variations of the legend, that is, there are scale-dependent layers and the legend may change when the map scale changes, create an image of the legend for each variation. The current map scale of the client should determine which static legend image should be returned.
When the served map contains an embedded image service, there is no workaround for legend requests directly made to ArcMap Server.