diff --git a/tools/binman/entry.py b/tools/binman/entry.py
index 5bf5be4794b4c92f87ff492df2dba0b29d963d70..b6f1b2c93fb195381d4d9d6130ab7cb2c72fa4f1 100644
--- a/tools/binman/entry.py
+++ b/tools/binman/entry.py
@@ -47,6 +47,8 @@ class Entry(object):
         offset: Offset of entry within the section, None if not known yet (in
             which case it will be calculated by Pack())
         size: Entry size in bytes, None if not known
+        pre_reset_size: size as it was before ResetForPack(). This allows us to
+            keep track of the size we started with and detect size changes
         uncomp_size: Size of uncompressed data in bytes, if the entry is
             compressed, else None
         contents_size: Size of contents in bytes, 0 by default
@@ -71,6 +73,7 @@ class Entry(object):
         self.name = node and (name_prefix + node.name) or 'none'
         self.offset = None
         self.size = None
+        self.pre_reset_size = None
         self.uncomp_size = None
         self.data = None
         self.contents_size = 0
@@ -314,6 +317,7 @@ class Entry(object):
         self.Detail('ResetForPack: offset %s->%s, size %s->%s' %
                     (ToHex(self.offset), ToHex(self.orig_offset),
                      ToHex(self.size), ToHex(self.orig_size)))
+        self.pre_reset_size = self.size
         self.offset = self.orig_offset
         self.size = self.orig_size
 
@@ -757,7 +761,10 @@ features to produce new behaviours.
             True if the data did not result in a resize of this entry, False if
                  the entry must be resized
         """
-        self.contents_size = self.size
+        if self.size is not None:
+            self.contents_size = self.size
+        else:
+            self.contents_size = self.pre_reset_size
         ok = self.ProcessContentsUpdate(data)
         self.Detail('WriteData: size=%x, ok=%s' % (len(data), ok))
         section_ok = self.section.WriteChildData(self)