Reduce max transactions per block in simtests (#20401)

## Description 

This makes it more likely to exercise full blocks in simtests.

## Test plan 

CI

---

## Release notes

Check each box that your changes affect. If none of the boxes relate to
your changes, release notes aren't required.

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that describes the impact of your changes that a user might notice and
any actions they must take to implement updates.

- [ ] Protocol: 
- [ ] Nodes (Validators and Full nodes): 
- [ ] Indexer: 
- [ ] JSON-RPC: 
- [ ] GraphQL: 
- [ ] CLI: 
- [ ] Rust SDK:
- [ ] REST API:

crates/sui-protocol-config/src/lib.rs

@@ -1631,14 +1631,20 @@ impl ProtocolConfig {
     }
 
     pub fn max_transactions_in_block_bytes(&self) -> u64 {
-        // Provide a default value if protocol config version is too low.
-        self.consensus_max_transactions_in_block_bytes
-            .unwrap_or(512 * 1024)
+        if cfg!(msim) {
+            256 * 1024
+        } else {
+            self.consensus_max_transactions_in_block_bytes
+                .unwrap_or(512 * 1024)
+        }
     }
 
     pub fn max_num_transactions_in_block(&self) -> u64 {
-        // 500 is the value used before this field is introduced.
-        self.consensus_max_num_transactions_in_block.unwrap_or(500)
+        if cfg!(msim) {
+            8
+        } else {
+            self.consensus_max_num_transactions_in_block.unwrap_or(512)
+        }
     }
 
     pub fn rethrow_serialization_type_layout_errors(&self) -> bool {