This study explored the impact of different tactical formations (TFs) on positional team behaviors in youth soccer during small sided games (SSGs). Eighteen U19 players participated in 7-a-side SSGs including goalkeepers, alternating between 2-3-1 and 3-1-2 TFs against a consistent opponent in the 3-3 TF. Positional data were collected with WIMU PRO GPS devices (Hudl, USA) to assess collective offensive and defensive dimensions that included teams´ width, length, height, the stretch index, and the surface area. The Mann-Whitney U test revealed that teams employing a 2-3-1 TF in offensive play exhibited increased height (p < 0.001; ES = 0.206), width (p = 0.006; ES = 0.113), and the surface area (p = 0.023; ES = 0.093) compared to the 3-1-2 TF. Conversely, defending with a 2-3-1 TF increased team height (p < 0.001; ES = 0.287) while decreasing length (p < 0.001; ES = 0.189), the surface area (p = 0.005; ES = 0.161), and the stretch index (p < 0.001; ES = 0.183) compared to the 3-1-2 TF. When attacking against a 2-3-1 TF, the offensive team experienced reduced height (p = 0.007; ES = 0.116) but an increased surface area (p < 0.001; ES = 0.241), width (p < 0.001; ES = 0.378) and the stretch index (p < 0.001; ES = 0.326) compared to the 3-1-2 TF. Finally, defending against a 2-3-1 TF resulted in decreased team length (p < 0.001; ES = 0.205), the surface area (p < 0.001; ES = 0.271) and the stretch index (p < 0.001; ES = 0.205) compared to defending against a 3-1-2 TF. Thus, coaches should acknowledge the significant role of TFs in modulating positional behaviors during SSGs, with relevant pedagogical implications for designing effective training sessions.