diff --git a/src/_zkapauthorizer/foolscap.py b/src/_zkapauthorizer/foolscap.py
index fd0eba52f179dacd80fec43f69f4e7f7e1593235..c9b865a66d01cb28d17c757f1971258aa34b7dc6 100644
--- a/src/_zkapauthorizer/foolscap.py
+++ b/src/_zkapauthorizer/foolscap.py
@@ -129,6 +129,9 @@ class RIPrivacyPassAuthorizedStorageServer(RemoteInterface):
         """
         Get the size of the given shares in the given storage index.  If there are
         no shares, ``None``.
+
+        The reported size may be larger than the actual share size if there
+        are more than four leases on the share.
         """
         return ChoiceOf(None, Offset)