I do love getting mathy, so bear with me.
tldr: Everything is correct and corruptors can deal a lot more damage than defenders.
Note that the numbers I'm using are all above lvl 20, before level 20 there are slightly different multipliers.
Defender have a damage multiplier of 0.65 ranged (and 0.55 melee) while corruptors have a damage multiplier of 0.75 for ranged and melee.
The defender inherent vigilance (
https://wiki.cityofheroesrebirth.com/wiki/Inherent_Powers#Vigilance) gives a defender a 30% damage bonus solo (20% with one teammate, 10% with two teammates, and 0% with three or more teammates). Therefore a base attack for a defender when solo deals 0.65 * 1.3 = 0.845, which is higher than the corruptor's 0.75.
However, their base multiplier is still lower. So let's suppose that a power were enhanced 100% (slightly above what you would get with 3 damage enhancements). Then defenders would effectively deal (0.65 * 2.3) = 1.495 and corruptors would deal (0.75 * 2.0) = 1.5. So at this point corruptors are already dealing more damage, albeit only slightly, but this will increase as there are more damage bonuses from buffs, set bonuses, eating a red inspiration, etc. Of course, when teamed the defender falls to 1.3 (=0.65*2.0).
Furthermore, corruptors have a damage cap of 500% while defenders have a damage cap of 400%. Therefore at the extreme (say capped due to Fulcrum Shift) corruptors can deal 5*0.75 = 3.75, while defenders can only deal 4*0.65 = 2.6.
Now add on top of that the corruptor inherent scourge (
https://wiki.cityofheroesrebirth.com/wiki/Scourge) which means that once enemies are below 50% health they have a chance to deal double damage and their damage increases even further relative to defenders (at least against hard targets that are going to suffer more than a couple hits before defeat).
As someone who primarily plays defenders, I agree that they are strong and their stronger buffs/debuffs is why I prefer them over corruptors. I think a lot of people don't realize that solo their damage output is comparable which is part of what leads to much higher number of corruptors than defenders. But if the only thing you care about is damage, corruptors have the ability to deal a lot more than defenders and given the breakdown on the server, I would say most people feel that this more than makes up for their reduced buffs and debuffs.