Fixing histogram size behavior within grid of graphics

I’m trying to put the finishing touches on a grid of figures and could use some help. I’m trying to keep the figures as clean as possible, so I only want frame ticks and labels on the edge figures. However, when I do this, the figures on the far left and right are shrunk relative to the middle ones. I’d love to know how to fix this.

Here’s the code and output.

(*set up scaling of population sizes*)
iVals = {1/2, 2, 8, 16};

(*create simulations *)
Table[
  Ne = 8*i;
  initialCount  = Ne; (*allele frequency = initialCount/(2 Ne) *)
  
  tMax = 5;
  nPops = 100;
  
  driftMatrix[i] = 
   Table[
     (*each row is the distribuiton of p at given generation *)
     
     NestList[
        RandomInteger[BinomialDistribution[2 Ne, #/(2 Ne)]] &, 
        initialCount, tMax + 1]/(2 Ne) // N, {i, nPops}] // 
    Transpose,
   {i, iVals}];

(*set up the bins*)
binWidth = {0.05};
binBoundaries = Range[ 0 - binWidth/2, 1 + binWidth/2, binWidth];
bin = binWidth;

(*choose which time steps to plot *)
tSteps = 10;
tVals = Range[0, 5];
rowVals = tVals + 1;

(*make the plots *)
plots = Table[
    Table[
     xTicks = If[j == Max[rowVals], Range[0, 1, 0.25], None];
     lTicks = 
      If[i == Min[iVals], {{10, Style["0.5", Smaller]}} , None];
     rTicks = 
      If[i == Max[iVals], {{10, Style["0.5", Smaller]}} , None];
     rFrame = If[i == Max[iVals], True , False];
     Histogram[
      driftMatrix[i][[j]], binBoundaries, "PDF", 
      PlotRange -> {{0, 1}, {0, 12}},
      Frame -> {{True, rFrame}, {True, False}},
      FrameTicks -> { 
        {lTicks, rTicks},
        {xTicks, None }
        },
      AspectRatio -> 1/3
      ], {j, rowVals}], {i, iVals}] // Transpose;

(*combine plots into a Grid*)

xlabels = 
  Text[Style[#, Medium]] & /@ (Join[8*iVals, {"Generation"}]);
ylabels = Text[Style[#, Medium]] & /@ (tVals);
Show[
 Labeled[
  Grid[
   Join[
    {xlabels},
    Transpose[
     Join[Transpose[plots], {ylabels}]
     ]
    ], Spacings -> {1, 0}
   ],
  {"Population Size", "Bin Frequency", 
   "Frequency of bw75 Allele"}, {Top, Left, Bottom},
  RotateLabel -> True],
 ImageSize -> 12*72
 ]

It’s subtle, but if you look you’ll see that the frame lines of the left and right most plots don’t match up with the interior ones.