A WebmasterWorld thread has a bunch of Google AdSense publishers complaining that certain Google AdSense ad units are resulting in the ad spots having way too much white space.
One publisher wrote:
I have a 300x600 adsense unit in my sidebar set to image/text and it's creating a lot of white space. What's happening is Google is occasionally showing just a single ad in the unit which sits at the top of the unit but leaves three quarters of the unit blank which looks funny since it keeps the content below it pushed all the way down.Is there a way to stop adsense from showing just one ad in this tall unit? It's ugly and annoying with all the white space.
I found one site where I can replicate this and here is a picture:

This does look pretty bad.
I am not sure how many sites are ad loads are impacted by this issue.
There is no word from Google on if they will fix this or if this is a serious issue or not.
Forum discussion at WebmasterWorld.

Comments:
Andrew Johnson
02/27/2013 02:00 pm
I'm having the same issue. My guess is that the 300X600 ad is still so new, that they don't have enough advertisers to fill it yet.
Fritz Oberhummer
02/27/2013 02:24 pm
wrapping a div-layer around it with same with/height and css-property overflow set to none helps ;)
Imran
02/27/2013 02:51 pm
You are absolutely right.
Madhav Tripathi
02/27/2013 05:33 pm
I don't think it is a bug. But sure it is annoying, a reserved space for 300x600.
Gonzalez Rivera
03/02/2013 06:43 am
I personally Hat from Google White Space unwanted space. This should not be occur during Google ads. SEM Consultant