Because, as !K_A! said, bigger sigs can be incredibly annoying - you try scrolling halfway down the page simply to get to the next post.

An extra 25px to the height won't be much (it's slightly smaller than the height of a standard taskbar), but I don't think there's much call for it - changing something to please one person isn't done very often, rather that person has to change to meet the standard. If enough people want it, however, then there's a good reason for it to be changed.