I've found that getting a horse to extend their stride and really step under themselves, move with impulsion, doing controlled and smooth downward transitions, and hill work (ride up and down hills) to be far more effective at building topline than 'long and low'. And that's coming from someone who practices dressage.
As far as the way the topline looks, some breeds actually just have tendency to keep more fat on their topline, so they look better than others even when not in work. My mom's QH could get no work at all for months and still look muscular in her topline. My horse (a mustang cross that has a saddlebred or TWH type look to her) requires hard work to keep that topline looking muscular and filled out. Even then, she won't have the topline of a QH. Basically part of it is where their individual physique has a tendency to store fat.
"You can do something wrong for thirty years and call yourself experienced, you can do something right for a week and experience more than someone who spent thirty years doing the wrong thing." ~WhattaTroublemaker