You need ~$1M worth of equipment to farm 80 acres (~$1M worth of land), but that same equipment can basically farm 800 acres (~10M worth of land). An equipment issue can destroy a years worth of work with 800 acres (e.g. frost damage from delays), but with 8000 acres you can have spares / avoid the loss with some overtime.
Fertilizer and pesticides aren't neatly contained. If you farm different crops than your neighbors, overspray can kill your yield (e.g. weeds spray for corn kills soybeans). Laws around who can grow/spray what and being big helps make that better