I would suspect your not using GD2 as your processor under admin global options or under upload options in admin your thumbnail size is not 150. This is an either or scenerio here. Each processor is going to build a thumbnail like you state it to be. Example you can use the new cropped thumbnail thing in Photopost 6.0 but that uses Imagemagick not GD2.
GD2 will work fine and build thumbnails using the size you have set in admin upload options for thumbnail threshold. If your getting some other size than that you must be using the uniform thumbnail feature and Imagemagick not GD2. I hope that clarifies the issue. Please reread my last reply and check the options I noted to see what processor your using and what size thumbnail your setting.
Not a problem let me know what you find out. Like I stated if your using GD2 under global options in admin your thumbnails will be what size your thumbnail threshold is set to in upload options in admin.