If you don't tell people you're new, people will assume you know what you are doing. The average tank does big pulls. The average healer can handle big pulls. The tank is under no obligation to tell you that she is going to do the thing she nearly always does and people are nearly always okay with. If you want something done differently, then it's your obligation to say something about it. I have many times seen tanks slow down when asked without complaining.

For one, I tank often, and I will always slow down if asked. But, if no one says anything, I will assume the party wants the dungeon done the way it's usually done, and that usually means big pulls.