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!
Damage calculator