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You must have set all the width etc. on the web page as fixed values. For example 700px wide. If you do this, then somebody with a 15" monitor will have a smaller resolution than somebody with a 21" monitor. So, the higher the resolution the smaller it looks.
You would need to make all the sizes and so on relative, so make them a percentage or something. Depending how complicated your website is, it could take some time to do it.
Or you could just change your monitor resolution to what you had your 15" at 🙂