Here are some damage calculations showing the damage that the same move does to each.
252 Atk Life Orb Arceus Extreme Speed vs. 0 HP / 0 Def Eviolite Clefairy: 192-227 (68.3 - 80.7%) -- guaranteed 2HKO
252 Atk Life Orb Arceus Extreme Speed vs. 0 HP / 0 Def Eviolite Jigglypuff: 331-391 (89.2 - 105.3%) -- 31.3% chance to OHKO
Clefairy takes the hit better than Jigglypuff does, so Clefairy is the bulkier of the two. Clefairy's Special Defence is better than its Defence, so I have no doubt that it will be bulkier on the special side as well. You can use the damage calculator linked below if you'd like to test it yourself!