I think Pete's comment that there seem to be more than one size of GHQ figures is quite accurate. And I don't deny av8r's assertion that a particular figure, or even set of figures, might not work well.
So we might consider some care in choosing which figures we mount on our vehicles. But I do not think the differences would somehow limit GHQ's abilities to create dedicated vehicle crew figures if they so choose.
I have seen the information posted here about the GHQ US Paras. They do seem to be notably larger than any of the GHQ figures I have bought over the years. So also I have seen comments posted here about the GHQ Vietnam infantry figures. They also seem to be larger. I do not own either, so can not speak from personal experience.
The ones I do own, and so with which I do have personal experience, do vary in size a bit. But I do not find that to be limiting, in my case. The largest variances I have seen, and have seen other posters mention, is between the the recent individual infantry packs, the individual artillery crew packs, and the packs of figures made to go into the halftracks. So ...
In particular, regarding this comment: "Vehicle crews, tank riders or commanders, will have to be in scale with the vehicle or they will look like the Shriners driving clown cars. When placed next to the existing infantry the crew(s) will look like child soldiers" I find that I disagree.

Here are (L-R) a GHQ Romanian infantry figure, a GHQ US Artillery figure, a GHQ US Armored Infantry gunner, and a UK-made brand-X 1/300 artillery crew figure with binocs. Yes it is clear that the armored infantry gunner (made specifically to fit in a GHQ vehicle) is a bit smaller. But it does not look like a child soldier to me, and I perceive no "clown car" appearance when I use the artillery figures in my vehicles.

I have also found that real people in 1-to-1 scale, also vary in size. Here I am doing the Spahpanzer shuffle with two friends who have occasionally posted on this forum (though not recently enough that most would recognize their nic's). I would observe that you can find significant variances in height even in a small population. Greater variance, in fact, than between the figures shown in the prior photo.

If we would like to see more credible sources on armor in our discussion, I offer this picture of authors Steven Zaloga and Michael Green, with a Panther for scale reference. I would observe that even among recognizable authorities on armor, one man's height might well reach to the top of a Panther's engine deck, while another's does not even come close to the top of the engine deck.

I often mix figures from different packs on one stand if it serves my purposes. Here among the units of my Romanian HQ, the center stand has one Romanian infantry figure (the grenadier, with arms bent to look like he is not involved in combat at the moment), one Italian officer figure (with inexplicable kepi, filed to look like a Romanian officer's bustina cap) and one US artillery figure (with the equally inexplicable range finder clipped to appear as binoculars).

Here I have some M6 "Fargo" US light tank destroyers from the other guys (bought and painted before GHQ announced they would make the same model). I have used mostly GHQ US Armored Infantry seated figures as the gun crews, but added a GHQ US Artillery figure standing to one of the models just for some variety. I do not find that the standing figure looks outsized compared to any similar population of men I have ever seen.
Just my own experiences and results. You may like your armies to be all exactly the same height. It just doesn't matter to me if I see some variance between figures. And I don't see GHQ being limited if they want to make crew figures that fit well (and easily) with their vehicles.