polygon size operations for orthogonal (Manhattanized) polygons

I've had an annoying problem that's been at the core of my Skill code base for a long time and has a fairly severe impact on my designs. In my designs, I draw many curvilinear shapes. However, the foundry that I design in does not accept designs with non-Manhattan polygons (e.g. every edge must be orthogonal to the global Cartesian coordinate system). Thus, I use a custom function to "Manhattanize" all of my polygons. 

At a later step in my design flow, I use size up and size down operations by using dbLayerSize() to perform some automatic DRC cleaning. This automatic DRC cleaning is a critical step in my design flow. The problem is that dbLayerSize() does not behave as I would like when there are Manhattanized shapes. I've attached a figure to illustrate. 

The pink is the size up polygon, and the smaller polygon is the original polygon. It's hard to see in the screenshot, but I measured the distance of the edge of each original polygon to the edge of the sized up polygon. The nominal size up is 0.1um, and the first rectangle shows exactly this distance between edges. However, the 2nd rectangle is rotated by 15 degrees -- thus requiring "Manhattanization" of the edges of the rectangle to meet foundry requirements. When the size operations is applied to this rectangle, the distance between edges is ~0.120um  -- or ~20nm larger than intended. The worst case is when the rectangle is rotated 45 degrees (shown in the 4th rectangle) which results in a ~sqrt(2)*0.100 = 141nm distance between edges. 

dbLayerSize() seems to size edges -- both horizontal and vertical. This is a very reasonable thing to do as long as all polygons are orthogonal to the global Cartesian grid. Any advice on how to get the desired behavior where the distance between edges of rotated polygons is (very nearly) the same as the polygon that's aligned to the Cartesian grid? 

Thanks,
M