Perhaps the tool is in a ButtonGroup? Tools in a ButtonGroup are always ImageOnly or ImageAndTextNormal regardless of the min/max size. I tested it out and setting the MaximumSize does use the LargeImage (and show it as a large tool) when in a RibbonGroup. e.g.

  <igRibbon:XamRibbon>
   <igRibbon:XamRibbon.Resources>
    <DrawingImage x:Key="smallImage">
     <DrawingImage.Drawing>
      <GeometryDrawing Brush="Red">
       <GeometryDrawing.Geometry>
        <RectangleGeometry Rect="0,0,16,16"/>
       </GeometryDrawing.Geometry>
      </GeometryDrawing>
     </DrawingImage.Drawing>
    </DrawingImage>

    <DrawingImage x:Key="largeImage">
     <DrawingImage.Drawing>
      <GeometryDrawing Brush="Green">
       <GeometryDrawing.Geometry>
        <RectangleGeometry Rect="0,0,32,32"/>
       </GeometryDrawing.Geometry>
      </GeometryDrawing>
     </DrawingImage.Drawing>
    </DrawingImage>
   </igRibbon:XamRibbon.Resources>
   <igRibbon:RibbonTabItem Header="Home">
    <igRibbon:RibbonGroup Caption="Clipboard">
     <igRibbon:RadioButtonTool 
      Caption="Large By Default"
      igRibbon:RibbonGroup.MaximumSize="ImageAndTextLarge"
      SmallImage="{StaticResource smallImage}"
      LargeImage="{StaticResource largeImage}"
      />
     <igRibbon:ButtonGroup>
      <igRibbon:RadioButtonTool
       Caption="Button Group Tools are never large"
       igRibbon:RibbonGroup.MaximumSize="ImageAndTextLarge"
       SmallImage="{StaticResource smallImage}"
       LargeImage="{StaticResource largeImage}"
      />
     </igRibbon:ButtonGroup>
    </igRibbon:RibbonGroup>
   </igRibbon:RibbonTabItem>
  </igRibbon:XamRibbon>