They are marketmakers. They make the market by jiggling price (like fishermen) unlike specialists who follow availability (more or less). Are 1,000,000 shares of this enough to get it to move in a day? Probably not unless it was all on one side or the other.
Heaviest/biggest ask orders occur on falls.
Making a market definately creates advantages for the maker, but like you say, they do have limits.
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