As Steve said, it's the depth of the profile bar on the left. Your signature is displayed below this, which makes it look more stretched out. Turn off signatures and it reduces overall post length.

I'm not going to be doing any further work on these forums in terms of development. It works as it is but is now quite out of date in terms of software version. Nothing new or altered will happen until the eventual planned upgrade to Xenforo when I have the time and money to work on it.