Hey @nnscr , welcome to Bitwarden community forums !
The reason i can guess for this is not much related to file-size or premium subscription but it is more related to the fact that , the current Bitwarden code doesn’t support viewing of on any external images in the app/web vault without first downloading the unencrypted copy of it on your local/cahced storage and then processing it.
Everybody may not like the icons to be left unencrypted while they are stored on the servers or in local cached storage
This can be inferred from the fact that the favicons(website icons) too are not encrypted on your local storage after they have been fetched from the icon server. Reference to it can be find here Though you could easily turn web-icons off if that concerns you.
You can read about it here Data Privacy for Website Icons | Bitwarden .
Also to be noted that the feature of viewing docs/images securely wouldn’t be trivial to implement keeping security in mind.
Till the time there is a safe way developed for it, i would rather request a pre-defined set of icons and colors that a user can chose from to set it as their folders/entry icons.
Currently as a workaround many of us use emojis in the folders’ name to make those folders easily distinguishable from others.
Hope my input would be helpful for this request.
Cheers 
Icon set reference image.
