Miss Marley was about 90-95 lbs at 8 months. But Capone's sister was 75 lbs at 8 months. I don't believe 80 lbs is abnormal for a Mastiff. Keep in mind a Mastiff can take between 2-3 years to reach there full grown potential, some Mastiff lines can take up to 4 years. A slow, steady growing puppy is a healthy puppy!
I don't support feeding Purina One for multiple reasons. I an new here and don't want to step on any toes, but two of my biggest reasons for being against Purina One is that there second ingredient on all the dry food is Corn. Corn is a filler and a source of allergies for so many dogs. My second reason, which is the most important of all is that they use Menadione Sodium Bisulfate Complex as an ingredient in there dog foods. Menadione Sodium Bisulfite Complex is a form of synthetic Vitamin K and is a questionable ingredient. Menadione Sodium Bisulfite is a banned ingredient in Europe due to the harmful side effects, it is a known for it's carcinogenic effects and it is toxic to kidneys, lungs, liver, mucous membranes. The FDA has banned it in over the counter supplements for humans due to the high toxicity levels, but have taken a more lax approach to this being in pet foods. Your pet can safely get Vitamin K in it's natural form by eating leafy green vegetables or a premium food that simply has natural Vitamin K or a natural source of Vitamin K (alfalfa, broccoli or kelp). There are plenty of pet foods out there without this questionable ingredient that I would consider first. (
http://www.sciencelab.com/xMSDS-Menadione sodium bisulfite-9924604)
IMHO Solid Gold is a much better food. It's first 2 ingredients are a meat source, it does not contain Menadione Sodium Bisulfate, it does contain a few fillers. But these fillers are a higher quality filler than corn and are a lot further down on the list of ingredients. I would recommend changing from Purina One to Solid Gold. If you are looking at comparing dog foods, this is a great site to reference:
http://www.dogfoodanalysis.com/dog_food_reviews/
I hope this helps, but it sounds like your puppy is growing just fine!