RMagick filesize giving strange output for different sized images

When the snippet below surprisingly giving the same output for the original and the resized image. Executed with rmagick (2.12.2) and ruby 1.8.7 (2008-08-11 patchlevel 72) [i486-linux] on Ubuntu Jaunty.

img = nil
File.open("~/rmagick/test/original.JPG", "r") { |f| img = f.read }
img = Magick::Image::from_blob(img).first
p img.filesize #=> 2875242, i.e 2.7 mb, similar when checked from file system
small = img.resize_to_fit(75, 75)
small.strip!
p small.filesize #=> 2875242 (again!), file system shows it's 2.7 kb
small.write("~/rmagick/test/s.jpg")

can any开发者_Go百科one please point the problem?

Thanks

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It would appear that the Magick::Image filesize property is only updated when the image is read from a file, or the file is saved.