It's not a client issue. It's a lemmy/instance-specific issue. Look at the following link in your browser, then tap the image to expand. You will see bad image quality. https://lemmy.world/comment/11323036
Edit: Works fine with my vger.social account. Looks like shit on lemmy.world.
Edit 2: Looks like shit on sh.itjust.works and lemmy.world. Which are both running very outdated 0.19.3. Looks great on feddit.org, vger.social, lemm.ee which are running up to date version of Lemmy (0.19.5).
Probably because Sync on your phone is connecting/downloading directly from Imgur, which is bad for privacy. Voyager uses the image cached on your instance, if possible, to avoid connecting to arbitrary hosts to load images when browsing your feed.
Oh ok. Well I probably like this better. If it had a toggle in options, I would toggle it this way. But it sounds like it should have a toggle in options at least. For all I know, maybe it does.