What you are recalling is false however... Extremely false:
Let's see an example:
Classic:
Kel'Thuzad has approximately 3,150,000 hit points.
Frost bolt single - 2 second cast. Inflicts 9,000 to 11,000 Frost damage
Frost bolt multiple - Inflicts 2,750 to 3,500 Frost damage and reduces the targets' movement speed for 4 sec.
Wotlk 25 man version
Kel'Thuzad has approximately 14,660,000 hit points.
Frostbolt — Inflicts 29250 to 30750 Frost damage
Frostbolt Volley — Inflicts 7200 to 8800 Frost damage to nearby enemies, reducing their movement speed for 4 sec.
Source:
https://wow.gamepedia.com/Kel%27Thuzad_(Classic)
https://wow.gamepedia.com/Kel%27Thuzad_(tactics)
Hell... even in 10 man KT (and all the other bosses) had twice the HP.
Dalinos, you are the one who's trolling. The 25 man version abilities hit 3 times harder on average, and bosses had 5 times more HP. Those are the facts, memories are something else entirely.