More or less, as far as the game goes.

IRL the distinction tends to be that "heavy" troops are those whose main, or only, tactical function is shock combat in more or less close order. The "lights" are those whose duties revolve more around mobility, cross-country maneuverability, ranged combat etc.

Obviously, the line between the two can be fuzzy in the extreme, especially in the case of high-end "light" units which can often rip rank-and-file "heavies" a proverbial new one up close and personal. But that, like the exact makeup of the units' equipement, is well beside the point.