Report missing optional dependencies as possible cause of file size increase

src/ocrmypdf/_validation.py

@@ -418,6 +418,20 @@ def report_output_file_size(options, input_file, output_file):
                 f"The argument --{arg.replace('_', '-')} was issued, causing transcoding."
             )
 
+    if options.optimize == 0:
+        reasons.append("Optimization was disabled.")
+    else:
+        image_optimizers = {
+            'jbig2': jbig2enc.available(),
+            'pngquant': pngquant.available(),
+        }
+        for name, available in image_optimizers.items():
+            if not available:
+                reasons.append(
+                    f"The optional dependency '{name}' was not found, so some image "
+                    f"optimizations could not be attempted."
+                )
+
     if reasons:
         explanation = "Possible reasons for this include:\n" + '\n'.join(reasons) + "\n"
     else:

tests/test_validation.py

@@ -117,11 +117,23 @@ def test_report_file_size(tmp_path, caplog):
     opts = make_opts()
     vd.report_output_file_size(opts, in_, out)
     assert caplog.text == ''
+    caplog.clear()
 
     os.truncate(in_, 25001)
     os.truncate(out, 50000)
     vd.report_output_file_size(opts, in_, out)
     assert 'No reason' in caplog.text
+    caplog.clear()
+
+    with patch('ocrmypdf._validation.jbig2enc.available', return_value=False):
+        vd.report_output_file_size(opts, in_, out)
+        assert 'optional dependency' in caplog.text
+    caplog.clear()
+
+    opts = make_opts(in_, out, optimize=0)
+    vd.report_output_file_size(opts, in_, out)
+    assert 'disabled' in caplog.text
+    caplog.clear()
 
 
 def test_false_action_store_true():