This is the typical problem with our court systems. Magwitch's basic assertion here is that the judges are discriminatory or that they apply sentences unjustly. Aren't the courts supposed to be just? Aren't the courts supposed to be the one place in a society in which all people are treated equally.
Magwitch would also assert that his involvement in their crimes was to a lesser extent than that of Compeyson.
Compeyson was having a pity on nothing and nobody.
So I begun wi' Compeyson, and a poor tool I was in his hands.
Magwitch here notes that Compeyson doesn't care about anyone but himself and is willing to sell out a friend or partner. Magwitch calling himself a "poor tool" demonstrates his understanding of his place with Compeyson.
Compeyson also insisted that they didn't communicate during their trials and that they received separate defenses.