This is common on any sites that use monotype font elements (pre/tt) without resizing. It's due partially to user agent defaults for monotype fonts and partially to the fonts x-height.

In your user CSS (under user settings) add something along the lines of:

code, pre, tt {
  font: 1.2em/1.5em "Courier New", Courier, monotype;
}
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You can play with the size/line-height values to suit your particular preference. For more information on CSS font control (and other CSS properties) visit W3C CSS2 - Fonts.

Note: If you're viewing PM on multiple platforms and are having trouble with your CSS font sizing rules. Probably due to the “quirks” mode (lack of) inheritance (caused by the absence of a URL in the !DOCTYPE) and the IE default font size bug (it uses “small” as default when it should be “medium”) you may want to use absolute pixel font sizes.