Answer to Question 1
Accretion is heat generated by the agglomeration of planetesimals when the planet was formed. Differentiation is heat generated by the energy released as dense objects fall toward the center of a planet during the formation of the core of a planet. Radioactive decay generates heat by releasing nuclear energy when an unstable (radioactive) isotope decays into a more stable element.
Answer to Question 2
Size is the most important factor in determining how rapidly a planet loses its internal heat. The larger a planet is, the deeper is the insulation that surrounds the core and keeps in the heat.