[horsa]

My limited engineering knowledge makes me think the size and speed of the turning sprocket is the big factor here. Not the length of the track itself. Assuming the tracks are both the same in terms of pitch and condition so they stay engaged with the sprocket teeth. So the differential regulating power to each side comes into play. A longer track would have more weight but since the vehicle rolls on it, that should also not be a factor.

In the end, going with what seems right prevailed. Make the track link count the same on each side and then tension each track so it looks the same on each side. If it continues to pull slightly to one side, I'd guess that would be due to rod tension for the brakes and steering which can be adjusted after a few turnbuckle revolutions to take pressure off the differential parts.