I believe IView.GetOutline method gets the bounding box of a view not a part as shown in the image below (red dotted line).
If you want to get the bounding box of a flat pattern part, you will need to get the bounding box sketch and then use sketch segment to get all lines in the bounding box sketch. You can then use ISketchSegment.GetLength method to get the length of each sketch segments. Have a look at the following example: Find Total Length of Sketch Segments in Sketch Example (VBA).
Hope this helps.