Your missing the point of the thread.
Unless you have driven a 1 series than you probably were under the impression that I was until I had a 1 series recently for a week. The interior room and seat width are so close to the 3 series that the difference is impercievable. I did say the backseat was smaller. But for 2 people upfront the difference is non-existent or not noticable.
The charts up above do not give any internal inside space measurements.
The point of the thread is given they can make a 1 series that is 400 or more pounds lighter than the m3 yet has almost identical front room space and seats, than why can't a smaller and lighter exterior m3 be built with a smaller external body and chassis like the 1 series.
People are demanding bigger inside the cabin and tech. I do not know if the M market is demanding bigger cars that have the same cabin room. That is the part I do not get.
If the 1 was quite a bit smaller inside than this would not be a question. It seems they got almost the same interior room in a car that is 400-500 pounds less and more nimble-really qualities the m3/4 strives for.