>Why does bone need to be heavier on a show dog?
It doesn't need to be heavier. It is usually the influence of the Australian and New Zealand lines that have made the bone heavier. Saying that, these dogs work sheep in their respective countries.
I think I know who you are talking about with the merle, if I am correct it is a small collie and although lightly built, looked balanced to me.
As someone said to her, she would do better under certain judges then others. For some unkown reason the merles do not seem to do very well on the show scene.
At least with the majority of show breeders their breeding stock is health tested. Unfortunately there are very few agility bred dogs with these tests. It is something I have never understood as you need to know if your dog has good hips at least for agility, a well bred balanced dog is certainly an advantage too. It may have something to do with cost as most are WSD's and will not sell for so much as a Border Collie.
>I KNOW there are some horrendously fat collies out there being shown and being placed
Well I attend most Championship shows and I have never seen them, a lot of coat, yes, heavier in bone, yes, but not horrendously fat.