diff --git a/substrate/abi/src/block.rs b/substrate/abi/src/block.rs
index a2d38a74..76c88666 100644
--- a/substrate/abi/src/block.rs
+++ b/substrate/abi/src/block.rs
@@ -131,7 +131,7 @@ impl Block {
   /// The size limit for a block.
   ///
   /// This is not enforced upon deserialization. Be careful accordingly.
-  pub const SIZE_LIMIT: usize = 1024 * 1024;
+  pub const SIZE_LIMIT: usize = 512 * 1024;
 }
 
 #[cfg(feature = "substrate")]