Understanding nix channel updates

Hi, whenever I add a new channel, like upgrading from 25.11 to 26.05, and then running nixos-rebuild dry-build --upgrade, the update is huge. There are dozens or even hundreds of derivations being updated. Often the required disk space extends 15GB. I noticed that many derivations are not even new versions. emacs was version 30.2 in 25.11 and is 30.2 in 26.05 still. However, the store paths are updated and gigabytes of space are required. Can someone explain this to me? And are there workarou...