That is because by default yt-dlp tries to download the best / better format available and (I take it the context of your downloads are YT) these are the best ones available.

You can customize which format to download by doing first:

yt-dlp -F <url>

Check what numbers are there, and then download the formats you want.

You can customize both the video format & rate, and same for sound, by using

yt-dlp #+# <url>

Where the first hash is the id of the video stream, and the 2nd hash is the id of the audio id.

Take into account that you'd probably need also ffmpeg available on the command line for this; that if you "mix" containers (Say, a webm video and an m4u audio stream) you-ll end up with an .mkv container; and finally, that all of the downloaded streams are themselves containers (.m4u, mp4 & webm).