Vivaldi on a diet
Vivaldi takes a lot space on the disk. But you can recover some of it. In C:\Program Files\Vivaldi\Application Vivaldi keeps older version you can safely delete (I hope final version won't behave like that). In C:\Program Files\Vivaldi\Application[current version]\Installer there's full installer, removing which saves lots of space. Also in C:\Users[user]\AppData\Local\Vivaldi\User Data\Default you can find various cache folders (some visible, other hidden down in other folders) that grow uncontrollably and eat your disk space.
Vivaldi only keeps one older version for backup in case something gets wrong during update. It's save to delete if new version works fine, but this behavior is not gonna change in final version.
The full installer is known. It'll probably get removed.
The cache is there for a reason…
Something went wrong (after update). And I had to reinstall. This backup is useless. And it contains all the junk collected using previous snapshot.
What's the purpose of Vivaldi's cache other than taking disk space? Hundreds of MB taken by it hasn't improved my browsing experience a bit.
What you're seeing is the awful caching habits of Chromium/Blink - hundreds of writes per minute to the HDD and enormous cache files, with nothing to show for it tin terms of browser efficiency. Caching and disk writes can and will be greatly improved. I was skeptical that this was possible on top of Blink, but it Opera can do it (and they have, to a large degree) then Vivaldi can do it, and do it better. Expect improvements down the road.