Pull out Bubble#activity_count

app/helpers/bubbles_helper.rb

@@ -8,9 +8,8 @@ module BubblesHelper
   end
 
   def bubble_size(bubble)
-    activity = bubble.boost_count + bubble.messages.comments.size
     rank =
-      case activity
+      case bubble.activity_count
       when 0..5   then "one"
       when 6..10  then "two"
       when 11..25 then "three"

app/models/bubble.rb

@@ -48,6 +48,10 @@ class Bubble < ApplicationRecord
     end
   end
 
+  def activity_count
+    boost_count + messages.comments.size
+  end
+
   private
     def set_default_title
       self.title = title.presence || "Untitled"