diff --git a/src/_pytest/assertion/util.py b/src/_pytest/assertion/util.py
index 4669278e6..684e7cce5 100644
--- a/src/_pytest/assertion/util.py
+++ b/src/_pytest/assertion/util.py
@@ -344,18 +344,19 @@ def _compare_eq_iterable(
 
     if _is_empty_vs_non_empty(left, right):
         left_formatting, right_formatting = _format_for_empty_and_non_empty(left, right)
+        lines_left = len(left_formatting)
+        lines_right = len(right_formatting)
     else:
         left_formatting = pprint.pformat(left).splitlines()
         right_formatting = pprint.pformat(right).splitlines()
 
-    # Re-format for different output lengths.
-    lines_left = len(left_formatting)
-    lines_right = len(right_formatting)
+        lines_left = len(left_formatting)
+        lines_right = len(right_formatting)
 
-    if lines_left != lines_right:
-        printer = PrettyPrinter()
-        left_formatting = printer.pformat(left).splitlines()
-        right_formatting = printer.pformat(right).splitlines()
+        if lines_left != lines_right:
+            printer = PrettyPrinter()
+            left_formatting = printer.pformat(left).splitlines()
+            right_formatting = printer.pformat(right).splitlines()
 
     if lines_left > 1 or lines_right > 1:
         _surrounding_parens_on_own_lines(left_formatting)
@@ -389,16 +390,15 @@ def _format_for_empty_and_non_empty(
 ) -> Tuple[List[str], List[str]]:
     if isinstance(left, (list, tuple)) and isinstance(right, (list, tuple)):
         if not left:
-            right_width = len(right[0]) + 4 if right else 80
+            right_width = max(len(s) + 4 for s in right)
             right_formatting = pprint.pformat(right, width=right_width).splitlines()
             left_formatting = pprint.pformat(left).splitlines()
         else:
-            left_width = len(left[0]) + 4 if left else 80
+            left_width = max(len(s) + 4 for s in left)
             left_formatting = pprint.pformat(left, width=left_width).splitlines()
             right_formatting = pprint.pformat(right).splitlines()
         return left_formatting, right_formatting
     else:
-        # Fall back to default formatting
         return pprint.pformat(left).splitlines(), pprint.pformat(right).splitlines()