diff --git a/synapse/util/caches/stream_change_cache.py b/synapse/util/caches/stream_change_cache.py
index 970488a19..a1aec7aa5 100644
--- a/synapse/util/caches/stream_change_cache.py
+++ b/synapse/util/caches/stream_change_cache.py
@@ -18,11 +18,15 @@ from synapse.util.caches import cache_counter, caches_by_name
 
 from blist import sorteddict
 import logging
+import os
 
 
 logger = logging.getLogger(__name__)
 
 
+CACHE_SIZE_FACTOR = float(os.environ.get("SYNAPSE_CACHE_FACTOR", 0.1))
+
+
 class StreamChangeCache(object):
     """Keeps track of the stream positions of the latest change in a set of entities.
 
@@ -33,7 +37,7 @@ class StreamChangeCache(object):
     old then the cache will simply return all given entities.
     """
     def __init__(self, name, current_stream_pos, max_size=10000, prefilled_cache={}):
-        self._max_size = max_size
+        self._max_size = int(max_size * CACHE_SIZE_FACTOR)
         self._entity_to_key = {}
         self._cache = sorteddict()
         self._earliest_known_stream_pos = current_stream_pos