It sounds like somebody has been feeding somebody a load of verbal diarrhea. You must have seen a dog being biled out, and they've told you it's margarine, and yoy believed it. Apart from making a dog violantly ill I can't see any benefits, a hard chaser will chase with a belly full of bricks and win anyway. To get a dogs price out to 15/1 or 20/1 it would take at least 5 runs, and the money to make it worthwhile would be more than the tote pool or a bookmaker would stand you for.
As for weight variation a dog can be up say .4, this is done pre kennelling, before the race the dogs are paraded, a dog can empty out and unless you weigh it how do you know the dogs weight.
As far as picking a winner if you sort out the trainers who can train and I don't neccessarily mean the leading trainers [the statistics are easily available] a dog that can run, in the right grade on the right side of the track you are a long way to being successful