This question was addressed in a recent paper by Dellal and colleagues (2011) who examined heart rate (HR), blood lactate ([La]), subjective perception of effort (rating of perceived exertion [RPE]), physical and technical performance of amateurs, and professional level players during various small-sided games (SSGs, 2 vs. 2, 3 vs. 3, and 4 vs. 4).
Their results showed that across the various SSGs, amateurs completed a lower percent of successful passes, recorded higher RPE and [La] values, lost a greater amount of ball possessions, and covered less total distance with respect to sprinting and high-intensity running. The HR responses, however, were similar when expressed as %HRmax.
This study shows that SSGs stress different aspects of fitness and technical performance in amateurs and elite players. Based on these findings, it seems that amateurs should supplement their fitness training with generic training as well.
Dellal, A, Hill-Haas, S, Lago-Penas, C, and Chamari, K. Small-sided games in soccer: Amateur vs. professional players' physiological responses, physical, and technical activities J Strength Cond Res 25(9): 2371-2381, 2011