Which Pokemon weigh less or the same when they evolve?

Solution 1:

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:

• Reddit
• Bulbapedia

Solution 2:

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)

Pokemon that remain the same weight:

• Gastly (0.2 lbs / 0.1 kg) to Haunter

Credit goes to Z9. for starting this list. This was sourced by manually going through the Pokemon list on Bulbapedia.