Return to Answer

realPlotRange = Module[ {padding = Total /@ (Options[#, PlotRangePadding][[-1, 2]] /. None -> 0), baserange = (#[[2]] - #[[1]] &) /@ PlotRange[#], range}, range = (baserange + padding) /. {a_ Scaled[b_] -> Scaled[a b], Scaled[a_] + Scaled[b_] -> Scaled[a + b]} /. {a_ + Scaled[b_] -> a/(1 - b)}; range] & 

Show[gr1, Epilog -> Inset[Show[gr2, ImageSize -> plotRangeRatio[gr2, gr1] printerPointsPlotRange[gr1] + (realImageDimensions[gr2] - printerPointsPlotRange[gr2]), AspectRatio -> (Last[#]/First[#] &)@ (plotRangeRatio[gr2, gr1] printerPointsPlotRange[gr1])], {0, 0}, {0, 0}, Automatic]}] 

realPlotRange = Module[ {padding = Total /@ (Options[#, PlotRangePadding][[-1, 2]] /. None -> 0), baserange = (#[[2]] - #[[1]] &) /@ PlotRange[#], range}, range = (baserange + padding) /. {a_ Scaled[b_] :> Scaled[a b], Scaled[a_] + Scaled[b_] :> Scaled[a + b]} /. {a_ + Scaled[b_] :> a/(1 - b)}; range] & 

Show[gr1, Epilog -> Inset[Show[gr2, ImageSize -> plotRangeRatio[gr2, gr1] printerPointsPlotRange[gr1] + (realImageDimensions[gr2] - printerPointsPlotRange[gr2]), AspectRatio -> (Last[#]/First[#] &)@ (plotRangeRatio[gr2, gr1] printerPointsPlotRange[gr1])], {0, 0}, {0, 0}, Automatic]] 

This approach does not account for PlotRangingPadding being different from zero. I'll update as soon as I can to remedy this.

As per the comment section of the OP I assume that the image padding is a constant number of printer points, though it is not necessarily known. I use the rasterize trick to obtain the size of the plotting range in printer points:

Ratio of the plot ranges of two plots:

plotRangeRatio = ((#[[2]] - #[[1]] &) /@ PlotRange[#1])/ ((#[[2]] - #[[1]] &) /@ PlotRange[#2]) & 

As per the comment section of the OP I assume that the image padding is a constant number of printer points, though it is not necessarily known. I use the rasterize trick to obtain the size of the plotting range in printer points:

I implement a modified version of PlotRange to account for PlotRangePadding.

realPlotRange = Module[ {padding = Total /@ (Options[#, PlotRangePadding][[-1, 2]] /. None -> 0), baserange = (#[[2]] - #[[1]] &) /@ PlotRange[#], range}, range = (baserange + padding) /.   {a_ Scaled[b_] -> Scaled[a b], Scaled[a_] + Scaled[b_] -> Scaled[a + b]} /.    {a_ + Scaled[b_] -> a/(1 - b)};  range] & 

It appears to fail for mixed specifications, such as {{1,Scaled[.1]},{Scaled[.02],Scaled[.02]}}. However if the left and right paddings are both given either in Scaled form or in the coordinates of the plot and the same holds for the top and bottom padding specs, the function works fine.

plotRangeRatio = realPlotRange[#1]/realPlotRange[#2] & 

As per the comment section of the OP I assume that the image padding is a constant number of printer points, though it is not necessarily known. I use the rasterize trick to obtain the size of the plotting range in printer points:

This approach does not account for PlotRangingPadding being different from zero. I'll update as soon as I can to remedy this.

As per the comment section of the OP I assume that the image padding is a constant number of printer points, though it is not necessarily known. I use the rasterize trick to obtain the size of the plotting range in printer points: