I agree with BrowserUK that because the image viewing software is trying to reduce the blockiness/pixelation of the image after zoom, by interpolating the 'missing' pixels after zoom using some interpolation algorithm based on neibouring pixels. This technique is generally Ok if not best for reducing the pixelation for photographs but not good at preserving sharp edges.
By the sound of it you probably don't want to enlarge an image physically. But if you do, you need a different image magnification algorithm all together to preserve the sharpness in the edges.
PS. There is a commercial product on Windows called S-Spline from Shortcut, which uses the company's S-Spline algorithm. I have tested the program and I was impressed by the quality of the enlarged image.
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