According to Emmanuel Pietriga (mail on list from 02/10/2010) the size of a graph that should be shown so that the whole window is filled, is calculated based on internal bounding boxes instead of considering the actual visible graph. As an effect of this, for (some?) graphs a large white border is displayed around the graph, meaning that the available space is not used efficiently. This can be seen in the left part of the attached screenshot (showing the overview window).
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Likely due to DPath approximate bounding box computation. Would need an example file to reproduce.