There are many politics in play in prison, simply because it is its own ecosystem. Things often occur when you stuff a large amount of individuals from different backgrounds with no respect for the law and lock them together. Hierarchies come into existence. It also does not help that many of these criminals are racist, and will separate each other based on race.
Regardless, like any social situation, there is always a dominant male in the group. However, being big does not automatically make you dominant. It is usually the most ferocious person that is the most dangerous one. These types of individuals come in all sizes.