diff --git a/changelog.d/17152.bugfix b/changelog.d/17152.bugfix
new file mode 100644
index 000000000..67aee9167
--- /dev/null
+++ b/changelog.d/17152.bugfix
@@ -0,0 +1 @@
+Fix bug where `StreamChangeCache` would not respect configured cache factors.
diff --git a/synapse/util/caches/stream_change_cache.py b/synapse/util/caches/stream_change_cache.py
index d8253bd94..2079ca789 100644
--- a/synapse/util/caches/stream_change_cache.py
+++ b/synapse/util/caches/stream_change_cache.py
@@ -115,7 +115,7 @@ class StreamChangeCache:
         """
         new_size = math.floor(self._original_max_size * factor)
         if new_size != self._max_size:
-            self.max_size = new_size
+            self._max_size = new_size
             self._evict()
             return True
         return False