Yes, a 4:3 sensor fits a circular image slightly more efficiently than a 3:2 sensor, but the gain is very small. For the same image-circle diameter, 4:3 captures about 4% more area than 3:2; a square sensor would be the most efficient, but still only modestly better.

In practice, that small geometric advantage does not make a meaningful difference to lens size or weight. Equivalent lens size is dominated by focal length, entrance pupil, and the image circle needed for the format. You can’t really “cheat physics”: a smaller-format lens can project a smaller image circle, but once you compare truly equivalent setups for field of view and total light gathered, the aspect-ratio efficiency difference is only a few percent.

So, while 4:3 is technically a bit more space-efficient than 3:2, it’s not enough to noticeably change real-world lens design. Choosing a system based on preferred aspect ratio, handling, and available lenses matters far more. Using a crop lens or crop mode on a full-frame body may use the projected circle differently, but it doesn’t create a major efficiency advantage by itself.