How to make a cow smaller (in BubbleChart3D plot)

Here's a solution that can show 1000 cows using 1.45MB.

The idea is to project the 3D cow onto a 2D plane and then use this as the ChartElement with Inset. The advantage to using Inset is the face will not rotate when rotating the graphic.

To project into 2D, I approximate the shape for the sake of speed and size of the mesh.

Options[approximateProjectedMeshRegion] = {RasterSize -> Automatic, 
  "NumPoints" -> Automatic, Method -> Automatic};

approximateProjectedMeshRegion[mr_, vp_, OptionsPattern[]] :=
  Block[{n, num, rand, θ, bds, xd, yd, xmin, ymin, raw, im},
    n = Replace[OptionValue[RasterSize], Except[_Integer?Positive] -> 90, {0}];
    num = Replace[OptionValue["NumPoints"], Except[_Integer?Positive] -> 200000, {0}];

    rand = RandomPoint[mr, num];
    rand = RotationMatrix[{vp, {0, 0, 1}}].Transpose[rand];

    θ = VectorAngle[(RotationMatrix[{vp, {0,0,1}}].{0,0,1})[[1;;2]], {0, 1}];
    rand = Transpose[RotationMatrix[-θ].rand[[1;;2]]];

    bds = MinMax /@ Transpose[rand];
    {xd, yd} = Abs[Subtract @@@ bds]/n;
    {xmin, ymin} = bds[[All, 1]];

    raw = Transpose[{
        Clip[n+1 - Round[Divide[#2 - ymin, yd]], {1, n}],
        Clip[Round[Divide[#1 - xmin, xd]], {1, n}]
      }& @@ Transpose[rand]
    ];

    im = Image[SparseArray[Thread[Union[raw] -> 1]]];

    ImageMesh[im, Method -> OptionValue[Method], DataRange -> bds]
  ]

The approximated cow:

bmr2D = approximateProjectedMeshRegion[cow, {1.3, -2.4, 2.}]

This is a poor looking approximation, but that's ok. With 1000 cows, you won't notice the jagged corners. Moreover, it's small in size:

facepts = MeshPrimitives[bmr2D, 2][[1, 1]];
ByteCount[Graphics3D[Inset[Graphics[Polygon[facepts]]]]]

1328

Now unfortunately BubbleChart3D does not seem to accept Inset inside ChartElements, so we need to workaround. What we'll do is manually place the cows and use the options returned from BubbleChart3D.

n = 1000;
chartopts = Options[BubbleChart3D[Thread[{
  data[[1 ;; n, 1]], data[[1 ;; n, 2]], data[[1 ;; n, 3]], ConstantArray[0.1, n]}]
]];

scene = Graphics3D[
  Inset[Graphics[Polygon[facepts]], #, Center, 15] & /@ data[[1 ;; n]], 
  chartopts, 
  ImageSize -> Large
];

Here's its size:

ByteCount[scene]

1458864

Compare this to the size of 1000 cow's from ExampleData:

1000 ByteCount[ExampleData[{"Geometry3D", "Cow"}]]

1399072000

And here's the scene:

scene

And like I said earlier, if you rotate the graphic the cows will face the same direction: