We don't play these modern clones at their intended resolution. DM was designed for the 320x200 resolution, but when played on an old monitor/CRT screen, the display was stretched vertically to fill the screen. The stretched view roughly translates to a resolution of 320x240, which means pixels are no longer square. Add in the curvature of the screen, the scanlines and other artifacts, and the visuals back then were very different from how we see them today on our perfect, crisp flat screens.
This is how it looked back in the day. Notice how the dungeon feels taller and less cramped than the modern view. And the limitations of CRT screens actually improve the look of the graphics in my opinion:

This is what we see today (note that this is doubled to 640x400 for ease of viewing)

The old size screen effect can be simulated on any modern screenshot by multiplying the 320x200 resolution up to 1600x1200. That's too big for the forum to display, so it'll squeeze it down and make it look a bit fuzzy, but you get the idea:

If wonder if clones could implement some kind of emulation of the effect, to stretch the output vertically and add some scanlines for that true retro feel;-)