diff --git a/Library/Homebrew/cmd/upgrade.rb b/Library/Homebrew/cmd/upgrade.rb
index 03fb307a09..89561c82b5 100644
--- a/Library/Homebrew/cmd/upgrade.rb
+++ b/Library/Homebrew/cmd/upgrade.rb
@@ -237,6 +237,10 @@ module Homebrew
           end
         }
 
+        # Build a unique list of formulae to size by including:
+        # 1. The original formulae to install.
+        # 2. Their outdated dependents (subject to pruning criteria).
+        # 3. Optionally, any installed formula that depends on one of these and is outdated.
         compute_sized_formulae = lambda { |f, check_dep: true, upgrade: true|
           sized_formulae = f.flat_map do |formula|
             # Always include the formula itself.