If you look at the BoxForm of the rasterized output in the Documentation Center by selecting it and pressing Ctrl+Shift+E, you'll see that GraphicsBox containing the rasterized plot has the option DefaultBaseStyle -> "ImageGraphics". So we can control magnification of such elements by adding to the Notebook's private stylesheet the following definition for the style "ImageGraphics":
Cell[StyleData["ImageGraphics"], Magnification->1]
This works great for any ordinary Notebook but applying this option to the Documentation Notebooks is tricky since they have the option Editable -> False and it isn't recommended to edit them. By default they have set StyleDefinitions -> FrontEnd`FileName[{"Wolfram"}, "Reference.nb", CharacterEncoding -> "UTF-8"] and hence their styles are defined in the stylesheet "Reference.nb" located in the folder
FileNameJoin[{$InstallationDirectory, "SystemFiles", "FrontEnd", "StyleSheets", "Wolfram"}]
You can make a copy of this stylesheet, remove the Editable -> False and Saveable -> False options and then edit the "ImageGraphics" style (located in the "FormatType and Automatic Styles" section). Then you can copy this file to the following folder and then restart Mathematica:
FileNameJoin[{$UserBaseDirectory, "SystemFiles", "FrontEnd", "StyleSheets", "Wolfram"}]
Stylesheets from this folder have higher priority than from the previous because it is located earlier in the StyleSheetPath global FrontEnd setting:
AbsoluteOptions[$FrontEnd, StyleSheetPath]
{StyleSheetPath -> {FrontEnd`FileName[{$UserBaseDirectory, "Autoload", _,
"FrontEnd", "StyleSheets"}],
FrontEnd`FileName[{$UserBaseDirectory, "Applications", _, "FrontEnd",
"StyleSheets"}],
FrontEnd`FileName[{$BaseDirectory, "Autoload", _, "FrontEnd",
"StyleSheets"}],
FrontEnd`FileName[{$BaseDirectory, "Applications", _, "FrontEnd",
"StyleSheets"}],
FrontEnd`FileName[{$InstallationDirectory, "AddOns", "Autoload", _,
"FrontEnd", "StyleSheets"}],
FrontEnd`FileName[{$InstallationDirectory, "AddOns", "Applications", _,
"FrontEnd", "StyleSheets"}],
FrontEnd`FileName[{$UserBaseDirectory, "SystemFiles", "FrontEnd",
"StyleSheets"}],
FrontEnd`FileName[{$BaseDirectory, "SystemFiles", "FrontEnd",
"StyleSheets"}],
FrontEnd`FileName[{$InstallationDirectory, "Configuration", "FrontEnd",
"StyleSheets"}],
FrontEnd`FileName[{$InstallationDirectory, "SystemFiles", "Components", _,
"FrontEnd", "StyleSheets"}],
FrontEnd`FileName[{$InstallationDirectory, "SystemFiles", "FrontEnd",
"StyleSheets"}],
FrontEnd`FileName[{"C:\\Program Files\\Wolfram Research\\Mathematica\\10.4\
\\SystemFiles\\Components\\MUnit\\FrontEnd", "StyleSheets"},
"PacletManager" -> True]}}
All the above operations can be completely automatized:
CreateDirectory[
FileNameJoin[{$UserBaseDirectory, "SystemFiles", "FrontEnd", "StyleSheets",
"Wolfram"}]];
CopyFile @@ (FileNameJoin[{#, "SystemFiles", "FrontEnd", "StyleSheets",
"Wolfram",
"Reference.nb"}] & /@ {$InstallationDirectory, $UserBaseDirectory});
nb = NotebookOpen[
FileNameJoin[{$UserBaseDirectory, "SystemFiles", "FrontEnd", "StyleSheets",
"Wolfram", "Reference.nb"}]];
SetOptions[nb, {Editable -> True, Saveable -> True}];
NotebookFind[nb, "ImageGraphics"];
cell = NotebookRead[nb];
If[TrueQ[First@cell === StyleData["ImageGraphics"]],
NotebookWrite[nb, Append[cell, Magnification -> 1], All]; NotebookSave[nb], $Failed]
(then restart Mathematica).
I have tested the above and it works! Screenshot:

SetOptions[ EvaluationNotebook[], {Magnification -> 1, GraphicsBoxOptions -> {BaseStyle -> Magnification -> 1} } ]but it seems I missed something. – Kuba Mar 11 '16 at 12:48Maginification -> 1.12I copied wrong number :) But it seems it doesn't work on Win. – Kuba Mar 11 '16 at 13:17GraphicsBoxOptions -> {BaseStyle -> Magnification -> 1}governs only images generated by Mathematica via plotting, and that it has no effect on images directly imported by Mathematica, such as that one attached above, fetched as an image file by the Help browser from the hard disk. It seems thatRasterBoxOptionsmay be relevant to the magnification of imported raster images, but my trial has been unsuccessful. – User18 Mar 13 '16 at 04:39SetOptions[ $FrontEndSession, GraphicsBoxOptions -> {BaseStyle -> Magnification -> 1}], but, of course, that affects *all* notebooks throughout the session. It does have the advantage of persistence whenever you change the doc page. (OSX 10.11.5, M V10.4.1.) – Michael E2 Jul 09 '16 at 23:43