Finnish Infantry advancing to a new position through a smoke screen - possibly in a training. The grand majority of Finnish infantry served in infantry regiments, which belonged to some certain division. Finnish Winter War era division and infantry regiment were much lighter and more weakly equipped armament-wise than their Soviet equivalents, hence the major improvement made by starting of Continuation War was giving them additional units, which gave them reasonable antitank- and antiaircraft-capacity, that they had been earlier lacking. Since the Soviets at the same time downsized their own divisions, in 1941 Finnish division actually ended up being larger than their Soviet equivalent. Around 1942 - 1943 Finnish Army downsized its divisions, but this was to release more manpower to home front, which was struggling to keep the military provided with weapons, food, ammunition and other equipment. While necessary, from purely military point of view this downsizing proved very poor idea, since it basically ruined the system in which way infantry regiments of division were used. After this downsizing divisions had only two infantry regiments, which made impossible the previous system of division typically using two of its regiments in frontline while keeping the third regiment as reserve. For all practical purposes this meant leaving divisions without their own reserves of useful size. At the same time Soviet infantry divisions developed to what could be described being close to brigade concept and relied more to firepower than manpower. During World War II Finnish Army also noted, that division was so large, that it was often difficult to command effectively. One solution for this problem was creating new (infantry) brigades around 1943 - 1944. These were units about size of infantry regiment, but with support units usually provided to Division. But the transition was not completed to many of the designed brigades and this new formation type didn't replace divisions in Finnish Army until after the war ended. Compared to their Soviet equivalent, Finnish divisions and infantry regiments were motorised in much lesser extent, even if the number of trucks in their use was considerably increased after Winter War (1939 - 1940).
Book "Finland at War: The Continuation and Lapland Wars 1941-45" by Vesa Nenye, Peter Munter, Toni Wirtanen and Chris Birks