that muscle is probably natural. i doubt that they sell little doggie supplements that are good for your dog. try excercising them more often and making them pull on a normal bases. also, if you live near a lake or a pond, swimming is good for a dogs's leg and chest muscles and also stamina.
Hi Big C from a fellow bulldogger. You have recieved a few good suggestions on food, so I will just go over a bit of conditioning. I weightpull my dogs as well as participate in Schutzhund, hangtime, flirtpole, etc.. The best way to build real defined muscle is not just pulling alone, but to alternate with aerobic excercise. I will pull distance with light weight about 4 days a week. (1 mile or more pulling a 25 pound logging chain) with jogging or free running sessions daily. I only pull heavyweight a few times a week (600-1000 or more pounds in sessions simulated to a real sanctioned pull) Pulling heavyweight all the time really does no good for muscle definition... like the worlds strongest man... they may be big bulky guys, but they are not "ripped" so to speak. Same applies to dogs.. dogs that just pull short heavy loads will bulk with muscle but they will not be defined. I keep things fun for the dogs by alternating excercises each day. Maybe pull some light weight for 15-20 minutes then play with the flirtpole for another 10. Next day we train in protection and as a release play ball for 10-15 minutes. Conditioning a dog takes time. You have to work very hard to have dogs with the muscle you see online. The first time I showed up at Schutzhund with my "fat" AB's I was practically laughed off the feild. I changed my regime and now I have some of the most well conditioned dogs around. I do have a few good links with weightpull conditioning programs. Feel free to email me and I can send the links to you and talk bulldogs. http://www.parkinsabs.com