You can spend CP to increase the damage of light and heavy attacks.
Dks have a skill that increases the damage of their heavy attacks by 40% and further if the target has low health. That way you can easiely hit low health targets for 20k+ with inferno staff or 2-hander.
The only way someone is going to hit for 13k on light attack is via the Sorcerer ultimate overload. It will appear on your death recap as "Overload Light Attack". Overload is an ultimate that transforms your weapon into two fists of lightning, and you can light attack for massive single target damage, or you can heavy attack for massive AoE damage in a cone. Every attack launched will consume some ultimate.
Heavy attacks however, it's perfectly possible for those to hit 13k out of stealth, thanks to sets/buffs like Molten Armaments, Undaunted Infilitrator/Unweaver and Bound Armaments. If you want to factor in stealth, the stealthed multiplier for a heavy attack far exceeds the multiplier of a regular ability, and values of 20k+ stealthed heavy attacks are not uncommon.
On top of all these ways to boost light/heavy attack damage, there is finally the Champion System, which has a whole tree (Atronarch) dedicated to boosting the damage of light/heavy attacks, in addition to the respective +damage type passives, namely Elemental Expert for Destuction Staves, Thaumaturge for the Restoration Staff and Mighty for all other weapons.
Dean the Cat
Somewhat Insane Puddicat
EU-PC Megaserver; Ebonheart Pact, Alliance Rank 34 This one hails from far Singapore, excuse this one for his high pings. He also apologizes for any formatting/spelling errors, as he tends to answer using a mobile device.
Insanity is the price of Knowledge. Herma-Mora and Sheogorath, this one bows before thee.
This one does not advocate for any class to be nerfed. There are far deeper underlying issues then a simple "Class Imbalance". The Champion System is the problem. Not classes.