Using the same body types works better for a game that is trying for a level of believable history in their alternate history.
Soldiers, even special characters, don't normally get field uniforms that are that different, and when they do, it's normally simply a change of fabric, or a few special pieces of equipment. A general in the US Army could add special extras to their uniform, like Patton's revolvers, but regular troops had general issue equipment. That's where the GI for US soldiers came from.
It becomes a logistical nightmare to try and equip troops in multiple ways. And while soldiers win battles, logistics wins wars.
I mentioned earlier that some game companies, and some players assembling their own models, use poses no real soldier would be found in just for a difference. That's fine if it's what you want, because they are your miniatures, but I like my toy soldiers to look like soldiers, so I don't mind models in basic, but realistic poses.
Dust gives us soldiers with a few special pieces of equipment, and we can paint soldiers with varied fabric for uniforms. I have my Axis light infantry in camo smocks, with the few that have helmet covers also in camo, and a few with other bits of camo. It gives my army a level of consistency, while also allowing differentiation for special troops.
Having special characters in normal uniforms, with at most some extra equipment, makes them look like real soldiers that are part of an army. If they field regular equipment, but better than usual, just swapping heads makes sense.
On the flip side, having special characters simply regular models with head swaps allows players who aren't in a position to get the special models, or who lose them, still able to field them effectively. With Stefan a flamethrower wielding officer, any head swap and a copy of the game data lets a player field him. You could without even doing the head swap, but it's good to let your opponent know where you special characters are. Either do a head swap, or give them a specific camo pattern for their uniform. Having more gear that is camo than other soldiers is an easy, yet historically accurate, way to model.
Having special characters fit in with other soldiers makes sense, because it also would make them less of a target for enemy snipers. The Soviets used special markings on command tanks until they realized the Germans shot them first. There's a reason soldiers are told not to salute officers when in combat areas. special characters are special, butthey're also not stupid. Some level of blending is why they grew to be powerful characters.