Clarify why the image is smaller

Co-authored-by: Marek <[email protected]>

CHANGELOG.md

@@ -8,7 +8,7 @@ and this project adheres to [Semantic Versioning](https://semver.org).
 ## [Zebra 1.4.0](https://github.com/ZcashFoundation/zebra/releases/tag/v1.4.0) - TODO: DATE
 
 Zebra's mining RPCs are now available in release builds. Our Docker images are significantly smaller,
-because the Zcash verification parameters are now built into the `zebrad` binary.
+because the smaller Zcash verification parameters are now built into the `zebrad` binary.
 TODO: rest of intro
 
 This release contains the following changes:
@@ -26,7 +26,8 @@ if your mining pool needs extra RPC methods or fields.
 ### Zcash Parameters in `zebrad` Binary
 
 `zebrad` now bundles zk-SNARK parameters directly into its binary. This increases the binary size
-by a few megabytes, but reduces the size of the Docker image by around 600 MB. The `zebrad download`
+by a few megabytes, but reduces the size of the Docker image by around 600 MB because
+the parameters don't contain the Sprout proving key anymore. The `zebrad download`
 command does nothing, so it has been removed.
 
 Previously, parameters were stored by default in these locations: