This can be reproduced using the release data topp:states and population style. To reproduce:
start two GeoServer instances, let's consider 8070 the local and 8080 the remote
configure topp:states as a remote layer
perform GetLegendGraphic operation:
The obtained legend image will look like the attaches `legend1.png`, it contains some extra space. The expected legend looks like 'legend2.png'.