The small, medium and large stat bags are all patterned to resemble a three-stage line of Pokémon which specialises in that stat.
The HP bags resemble Azurill, Marill and Azumarill;
Attack bags resemble Machop, Machoke and Machamp;
Defense bags resemble Aron, Lairon and Aggron;
Special Attack bags resemble Litwick, Lampent and Chandelure;
Special Defense bags resemble Flabébé, Floette and Florges;
Speed bags resemble Zubat, Golbat and Crobat.
Hope I help!