Grid ItemSize is smaller than expected

If you set ImageSizeMultipliers -> 1 then the graphics will not be downsized when appear inside list-like constructs:

Style[
     Grid[{{200!}, {MatrixPlot@IdentityMatrix@100}}, ItemSize -> Automatic],   
     ImageSizeMultipliers -> 1]

Alternatively,

Grid[{{200!}, {MatrixPlot@IdentityMatrix@100}}, ItemSize -> Automatic,
  BaseStyle -> ImageSizeMultipliers -> 1]

or

Grid[{{200!}, {MatrixPlot@IdentityMatrix@100}}, ItemSize -> Automatic,
  ItemStyle -> ImageSizeMultipliers -> 1]

give the same output.

Although he puts it somewhat obliquely, Kuba is giving you good advice in his comment. When you place a graphics object into a Grid or Column construct and care even a little about its size, explicitly make the setting of the ImageSize option in the graphics object.