diff --git a/lib/matplotlib/backends/backend_wx.py b/lib/matplotlib/backends/backend_wx.py
index d2fd3c5218ab..05e20a0d93f1 100644
--- a/lib/matplotlib/backends/backend_wx.py
+++ b/lib/matplotlib/backends/backend_wx.py
@@ -947,8 +947,21 @@ def _onSize(self, evt):
         """
 
         DEBUG_MSG("_onSize()", 2, self)
+        sz = self.GetParent().GetSizer()
+        if sz:
+            si = sz.GetItem(self)
+        if sz and si and not si.Proportion and not si.Flag & wx.EXPAND:
+            # managed by a sizer, but with a fixed size
+            size = self.GetMinSize()
+        else:
+            # variable size
+            size = self.GetClientSize()
+        if getattr(self, "_width", None):
+            if size == (self._width, self._height):
+                # no change in size
+                return
+        self._width, self._height = size
         # Create a new, correctly sized bitmap
-        self._width, self._height = self.GetClientSize()
         self.bitmap = wxc.EmptyBitmap(self._width, self._height)
 
         self._isDrawn = False