[webs:sketch] do not match bitmap canvases that are too big

lib/web_ui/lib/src/engine/html/picture.dart

diff --git a/lib/web_ui/lib/src/engine/html/picture.dart b/lib/web_ui/lib/src/engine/html/picture.dart
index da6f988..e16c252 100644
--- a/lib/web_ui/lib/src/engine/html/picture.dart
+++ b/lib/web_ui/lib/src/engine/html/picture.dart
@@ -427,6 +427,12 @@
         final double pixelCountRatio = newPixelCount / oldPixelCount;
         assert(0 <= pixelCountRatio && pixelCountRatio <= 1.0,
             'Invalid pixel count ratio $pixelCountRatio');
+        
+        if (pixelCountRatio < 0.25) {
+          // The old canvas has more than 4x the number of pixels we need. It's best
+          // to let that canvas be recycled and allocate a smaller one.
+          return 1.0;
+        }
         return 1.0 - pixelCountRatio;
       }
     }