Isn’t the problem here that your 6mm figures are actually pretty big?
They are bigger than most to be fair, averaging about 6.5 to 7mm.
But here is another 6mm figure (Exodus Wars as was) and again, for me, the figure is a normal 6mm figure but placed next to the car seems to be too big to fit in it.
I also think width is an issue around this..
Infantry can be the right height but not the right width.
I have just measured my forearm and it is just over 4″ wide.
Which is 1/3rd of a foot.
Sooo using the assumption that 1mm = 1 foot the arms on a 6mm figure should be about 0.33mm wide.
This is simply not possible.
I have just measured some of my 6mm troops and they are about 3mm wide at the shoulders.
I am not aware of many people that are 3 foot wide, the average door width being about 2.5 feet.