document.optimizeresources(optVals) throwing out of memory exception

I am using the below code to optimize (shrink) existing large PDF files in VS2012 using vb.net:

        Dim aspDoc As New Aspose.Pdf.Document(strFN)
        Dim optVals As New Aspose.Pdf.Document.OptimizationOptions

        optVals.CompressImages = True
        optVals.ImageQuality = 22
        optVals.RemoveUnusedObjects = True
        optVals.RemoveUnusedStreams = True
        optVals.LinkDuplcateStreams = True
        aspDoc.OptimizeSize = True
        Try
            aspDoc.OptimizeResources(optVals)
        Catch ex As Exception
            MsgBox("Error optimizing resources!", MsgBoxStyle.Exclamation, "")
        End Try

        aspDoc.Save(strFN)

        aspDoc.Dispose()
        GC.Collect()
        aspDoc = Nothing
        GC.Collect()

It works just fine on the 1st PDF file, shrinks down from 50+ MB to 5 MB but throws an OUT OF MEMORY exception on any subsequent attempts on other PDF files of the same magnitude during the same run-time session. So only the 1st large PDF file gets optimized without error. See below error:

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When I check performance in task manager it shows 5.38 GIG used out of 8 GIG total usuable memory. Why?