Little update:
I used a rotary tool to make some room in the "footregion" of the buggies, with... mixed results
More importantly I had to accept that those pilots, at least as long as they are sitting on their seats, won't go in there. Even if I solved the "footproblem", they are also very high. At least in the big buggy this will mean that the head pops up out of the rollcage which looks incredibly dumb.
Therefore I removed the seats from the pilots, together with the toes of the small buggy pilot on the whole feet of the big buggy dude. It's an ugly solution, but it is one that works
Now they fit were they belong:
Sidenote: the tracks of the halftrack version of the big buggy were also weird. They consisted of two parts, the main body and the stretch of tracks on the ground as separate part. But those again did not really fit. The trackpart had guiding teeth, but there was no room for all of those in the body, so I had to shorten/remove some of them. Also the tracklinks had the connectors to go together but no room for the corresponding connector. So the track part was just to long and had also to be shortened.
All in all: as someone who was very satisfied with Mortian Ogres and still is satisfied with the design and details on the buggies I'm a bit disappointed with these little mistakes. More than all I'm curious how those slipped the attention during design/production. It's not even a minor thing, you see it at first glance when dryfitting the parts. And we are not talking about fractures of millimeters but whole milimeters.