Each box has a minimum width of 2 meters, which means the boxes on each end are ~21 meters away from the center. However, any yahoo setting up the course should create a curved start line so that all boxes have the same distance to the first turn.
However, even with a straight line start:
* if the course runs out 100m along the center line before any turns, those on the outside run sqrt(100^2+21^2)/100 or 1.0218 times further (~2.2 meters).
* if the course runs out 200m along the center line before any turns, those on the outside run sqrt(200^2+21^2)/100 or 1.005497 times further (~1.1 meters).
* if the course runs out at least 400m along the center line before any turns (as any large invitational should), then those on the outside run sqrt(400^2 +21^2)/100 or 1.001377 times further (~0.55 meters).
The starting line for a large invitational should be curved, but even if it's not, the advantage for teams in the middle is relatively small and may be negligible when you consider the congestion in the middle of the field.