Tank carry capacity

[Imp]

What are the criteria for a tank having a carry capacity, normaly of 13 or 6 for a light.
I assumed Reactive armour or VIRSS would negate as would a combination of TI & Laser rangefinder due to not obstructing equipment.
This holds true for most but some for example Leo series can still carry troops at a pinch. Why if the likes of a Chally or Abrams cant? Applies to upgraded M series of tanks to & probably some others. Certainly with the modern stuff which is pretty fast think should err on the side of no capacity, I would not like to be the guy hanging on as the player moves it flat out cross country, bit diffrent from sedate travel down a road. The main time it gets used at least by me is flat out travel to extract or possibly at start of the game before move to contact. Or end game to move by a flag on vehicle unfreindly terrain so again probably going flat out. After all if your tanks in combat mode infantry can pretty much keep up & its a lot safer for them.