I discovered recently this when I edited my first 4K video (I previously worked with HD video). With HD, my titles were typically in 64-72 point Ariel font, which looked fine on the screen. However, when I moved to 4K video, the 72 point titles were tiny and had to double the point size to make them comparable with HD.
At first I thought this was a bug, but then read in the online Help,
“The physical size of the font depends on the resolution of the image. A capital letter in 72‑point text is approximately 1‑inch high in an image that is 72 ppi. Higher resolutions reduce a given text point size because the pixels are packed more tightly in higher resolution images.”
Apparently PrE was designed this way. Regardless, does anyone know why? To me, it would be much more user friendly if font size were relative to the total image rather than the number of pixels. For example, a font size of 100 might represent 10% of the vertical image dimension, regardless of the resolution, so one could fill the screen with 10 lines of text. Likewise, a font size of 50 would occupy 5% of the vertical, and the screen would accommodate 20 lines.
I know that font size “point” standards are historical and resulted from the original printing presses where a point was 1/72 of an inch. This carried over to word processing and was successful because page sizes were usually standard (e.g., 8.5 x 11 inches). However, with video (or still photos) of widely different resolutions, Adobe’s interpretation doesn’t work well.
It's not that the pixels are denser in higher resolution. It's that the overall measurement of your video frame, in pixels, is much larger.
So if your standard definition video frame is only 480 pixels tall, then a 400 point font would practically fill the screen, top to bottom. But if your high-def video frame is 1080 pixels tall, your 400 point font would only be 40% as tall as the frame. Likewise, if your 4K video is 2100 or so pixels tall, your 400 point font is only about 20% as tall as the frame.
Pixels per inch or ppi doesn't mean anything in the video world. There are no inches. Just pixels. And everything (video, graphics, photos and fonts) have to be measured relative to pixels.