For the first sentence, it should be "I ate large pieces of steak that [were] very difficult to chew." Were is used for this sentence because the subject is pieces; "of steak" would be a prepositional phrase.
For the second sentence, it should be "You are a litterbug. You threw garbage [on] the ground." Consider the ground being a surface that you're putting something on.
For the last sentence, all sentences are correct. The last two have the same meaning, but the first one's meaning is slightly different. The last two sentences focus on the number of motorcycle owners (per capita, ownership), but the first sentence focuses on the number of motorcycles. "Density" means the number of something (people, objects, etc.) per area. It doesn't necessarily connect the number of motorcycles with the number of motorcycle owners.