Speaking to a few far more learned infantry warrant officer types the biggest issue regarding infantry squad size is moving them around the battlefield. Due to rail movement, and pesky things like small bridges and goat tracks - highways in some third world ####e hole, the vehicles have a width restriction. This means seven to ten soldiers with kit can squeeze into an APC/IFV/IMV etc. The marines have those big AAV7 'ordnance magnets' and the new Chinese ZDB05 amphibious infantry fighting vehicle is another large 'ordnance magnet' with lovely slab sides and thin armour.