Since you're saying you see white space all around the quick reply box, I'll assume you're using an ad blocker.
Basically, up until the other day, below the listing of threads were a Google AdSense ad and a rectangle banner ad side-by-side. Below it was the Quick Reply.
However, I received numerous complaints that you had to scroll past two ads in order to find the quick reply box. People also complained that they really wanted the quick reply box to be within the same scroll view of the thread post they were replying to. I also just recently received a complaint that there was "wall to wall" advertising with the two rectangle ads side by side and the skyscraper in the sidebar.
To resolve all of those issues, I went ahead and replaced the Google AdSense ad with the Quick Reply box. This A) Removed one of the ads B) Brought the quick reply box up right below the thread listing C) Makes thread pages load faster with one less ad D) Will hopefully encourage more replies for those times when someone just wants to post a quick comment but doesn't want to bother scrolling down past the ads, or doesn't even see the quick reply box because of the ads.
From a monetary perspective, the amount of money lost by the Google ad will hopefully be made up by the increased view rate of the existing ad, which is in view while people type a quick reply.
The quick reply area has always been tailored for just that ... quick replies ... and I suspect anyone posting long code tags would use the full editor anyways, as it includes the code wysiwyg buttons and is even bigger than the quickreply used to be.
I must say that the size of the quick post reply box is more annoying than having to scroll past an advertisement to reply.
But since this change was the result of user request, I guess we will just have to train ourselves to click the advanced reply button. At any case, we have to click it after the reply gets more than 5 lines in length..