In section 4.8, Energy dissipation in current flow, of Purcell and Morin's Electricity in Magnetism, the expression for the power expended by a resistor is derived. The sections includes the following discussion:
The energy thus expended shows up eventually as heat. In our model of ionic conduction, the way this comes about is quite clear. The ion acquires some extra kinetic energy, as well as momentum, between collisions. A collision, or at most a few collisions, redirects its momentum at random but does not necessarily restore the kinetic energy to normal. For that to happen the ion has to transfer kinetic energy to the obstacle that deflects it. Suppose the charge carrier has a considerably smaller mass than the neutral atom it collides with. The average transfer of kinetic energy is small when a billiard ball collides with a bowling ball. Therefore the ion (billiard ball) will continue to accumulate extra energy until its average kinetic energy is so high that its average loss of energy in a collision equals the amount gained between collisions. In this way, by first “heating up” the charge carriers themselves, the work done by the electrical force driving the charge carriers is eventually passed on to the rest of the medium as random kinetic energy, or heat.
I may be missing out on something elementary but I'm unable to understand the concepts in the paragraph related to energy transfer in the collisions.