Which Pokemon weigh less or the same when they evolve?

Most Pokemon gain weight (and height) when they evolve. There are some exceptions to this, for example Gloom to Bellossom, and Cosmeom to either Solgaleo or Lunala. Which Pokemon lose weight or retain their weight when evolving?

There are some lists online, but they are all incomplete.

2 Answers

Pokemon that lose weight:

Gloom (29 lbs / 8.6 kg) → Bellossom (12.8 lbs / 5.8 kg)
Porygon (80.5 lbs. / 36.5 kg) → Porygon2 (71.6 lbs. / 32.5 kg)
Shelgon (243.6 lbs. / 110.5 kg ) → Salamence (226.2 lbs. / 102.6 kg) (Thanks to Sumurai8 & MBorg)
Nincada (12.1 lbs. / 5.5 kg) → Shedninja (2.6 lbs. / 1.2 kg)

Pokemon that lose height:

Gloom (2'7" / 80cm) to Bellossom (1'4" / 40cm)
Dragonair (13'01" / 4m long) → Dragonite (7'03" / 2.2m tall)
Porygon (2'07" / 80cm) → Porygon2 (2'00" / 60cm)
Haunter (5'03" / 1.6 m) → Gengar (4'11" / 1.5 m)

Notes

Gengar (89.3 lbs. / 40.5 kg ) weighs more than Haunter (0.2 lbs. / 0.1 kg) (Thanks MBorg)
Porygon2 gains some of the weight back for Porygon Z.
Nincada to Shedninja loses about 10 lbs, but there are extenuating circumstances.
This list is most likely incomplete: please edit if you know others

Sources:

The Pokemon that lose weight when they evolve are:

Gloom (29 lbs / 8.6 kg) to Bellossom (12.8 lbs / 5.8 kg)
Kadabra (124.6 lbs / 56.5 kg) to Alakazam (105.8 lbs / 48.0 kg). Alakazam's weight does not change when it Mega Evolves
Porygon (80.5 lbs / 36.5 kg) to Porygon2 (71.6 lbs / 32.5 kg). Porygon-Z has weight in between the two at 75.0 lbs / 34.0 kg
Though not strictly an evolution, Mewtwo's weight (269.0 lbs / 122.0kg) decreases when Mega Evolving to Mega Mewtwo Y (72.8 lbs / 33.0 kg)
Galarian Corsola (1.1 lbs / 0.5 kg) to Cursola (0.9 lbs / 0.4 kg). Normal Corsola weighs more than both of them (11.0 lbs / 5.0 kg)
Though not strictly an evolution, Nincada (12.1 lbs / 5.5 kg) to Shedninja (2.6 lbs / 1.2 kg)
Clamperl (115.7 lbs / 52.5 kg) to either Huntail (59.5 lbs / 27.0 kg) or Gorebyss (49.8 lbs / 22.6 kg)
Shelgon (243.6 lbs / 110.5 kg ) to Salamence (226.2 lbs / 102.6 kg)
Gligar (142.9 lbs / 64.8 kg) to Gliscor (93.7 lb / 42.5 kg). This was the one that surprised me the most.
Though not an evolution, Phione (6.8 lbs / 3.1 kg) weighs more than Manaphy (3.1 lbs / 1.4 kg)
Type Null (255.7 lbs / 120.5 kg) to Silvally (221.6 lbs / 100.5 kg)
Cosmeom (2204.4 lbs / 999.9 kg) to Solgaleo (507.1 lbs / 230.kg) or Lunala (264.6 lbs / 120.0 kg)