On Steam: No. Only one build is installed at a time, and Steam downloads just the difference between your current build and the Experimental one — so the download stays small and quick. Switching back to the default branch works the same way.
On Xbox: Only the first time you install each build. The Experimental build is installed as a separate app alongside the stable version, so its first install requires a full download. Once both are on your console, switching between them requires no re-download at all — you'll only receive each build's own updates, which are incremental just like on Steam.