Small jig sizes imitate small baitfishes and aquatic insects, it's sort of the match the hatch concept. Since panfish is the primary fish sought during hardwater season, it makes sense to do so. Somtimes going with a larger heavier jig size will allow one to get the presentation down in deep water faster to maximize small windows of opportunity when a hot bite is on. At other times the larger lure profiles serves the purpose of attraction fish. Serves better if you're targeting walleyes.

Generally speaking, use the size of jig that gets you to the desired depth and a size of jig in proportion to your bait presentation (i.e. a 1/8oz. jig with a large minnow is basically useless and a 1/2oz. jig with a crawler is usually overkill). As Bob mentioned below, use enough weight to get to the bottom and then experiment with colors. Glow white was the best color for us the last couple of years!