The designers of FogBugz decided that the main bug description area should always be a fixed width, rather than widening automatically to fill the browser width.

This is not always the most popular decision; many customers feel like it wastes screen real estate. We did it that way because there's a lot of research about the readability of text. Columns that are too wide are hard to read, because it takes a long time for the eye to move from the end of each line to the beginning of the next line, and it's easy to accidentally skip a line or read a line twice when the line gets really long. Graphic designers have known this for a long time and even have guidelines for how wide columns of text should be.

If this still drives you crazy, Richard Thombs developed a little GreaseMonkey script to force FogBugz 6.0 to use more of the screen to display bug details. (Thanks, Richard!)