diff -r 21a9cace4bbf -r 42f59d3f714d mercurial/revlogutils/deltas.py
--- a/mercurial/revlogutils/deltas.py	Mon Dec 17 10:37:22 2018 +0100
+++ b/mercurial/revlogutils/deltas.py	Mon Dec 17 10:42:19 2018 +0100
@@ -601,6 +601,11 @@
 
     return True
 
+# If a revision's full text is that much bigger than a base candidate full
+# text's, it is very unlikely that it will produce a valid delta. We no longer
+# consider these candidates.
+LIMIT_BASE2TEXT = 50
+
 def _candidategroups(revlog, textlen, p1, p2, cachedelta):
     """Provides group of revision to be tested as delta base
 
@@ -614,6 +619,7 @@
 
     deltalength = revlog.length
     deltaparent = revlog.deltaparent
+    sparse = revlog._sparserevlog
     good = None
 
     deltas_limit = textlen * LIMIT_DELTA2TEXT
@@ -644,6 +650,8 @@
             # filter out delta base that will never produce good delta
             if deltas_limit < revlog.length(rev):
                 continue
+            if sparse and revlog.rawsize(rev) < (textlen // LIMIT_BASE2TEXT):
+                continue
             # no delta for rawtext-changing revs (see "candelta" for why)
             if revlog.flags(rev) & REVIDX_RAWTEXT_CHANGING_FLAGS:
                 continue