That's a good question!
Did anyone ever compare any screenshots to the original (1971 or 1978) 16/35mm film to check for subsequently released media (Beta, VHS, LD, or more likely DVD and BD) mis-proportions? Here are some full-sized 16mm frames (just ignore the demonstrated restoration) with which to compare:
http://originaltrilogy.com/forum/topic.cfm/THX-1138-preservations-Italian-Cut-available-see-1st-post/post/666433/#TopicPost666433
Those captures are from the SEN-talking-to-the-children sequence, just before THX & SRT hopping into jet-cars for their escape, at the last 15-20 minutes of the movie.