The reason it is using so much memory is because of the size of your image. It will use 5058 x 9251 x 16 bytes per pixel = 748664928 bytes = 740MB per page. At some point ImageMagick will switch to disk instead of memory. This will dramatically decrease the performance of your application. The default values that are being used for this will change in the next version of Magick.NET. Contact me through CodePlex if you are interested in using a development version of the x64 .NET 4.0 build that includes these fixes.
You can also handle each page separately instead of reading them all at once. You can specify the page with the FrameIndex property of MagickReadSettings. This will reduce the total amount of memory that is being used.
You can also handle each page separately instead of reading them all at once. You can specify the page with the FrameIndex property of MagickReadSettings. This will reduce the total amount of memory that is being used.