It doesn't look good when it's so big.

Unfortunately the GTK+ size model is not really good enough to get
exactly the behavior we want, so we have to settle for an
insufficiently-large address bar at large window sizes. It's not the end
of the world, but it would be good to improve this so that the location
entry can expand to fill larger windows. Unfortunately the only way I
know of to do this would be to set a huge amount of margin, which
results in a huge minimum size for the window and breaks shell tiling.