Winter is like any other season: Spring, which is broken up into Pre-Spawn, Spawn, and Post-Spawn; Summer, which is broken up into Summer, and Peak Summer; Fall, which is broken up into Early Fall, Fall, and Late Fall. Winter can also be broken up into sub-categories; Early winter, Peak Winter, and Late Winter.
Early and Late winter are transition periods, and depending on where you live in the country, and what type of winter you are experiencing, can determine what lure sizes are best. Typically, early and late winter, bass will still go after bigger lures, where during the peak (when the water is the coldest), smaller finesse lures rule. In the North, Peak winter can last two to three months; in the South, peak winter may only be 2-3 weeks.
Another reason is; most people feel comfortable going small, because smaller lures catch numbers, and when fishing is tough, it simply makes sense to go for catching anything, versus catching nothing.
