Theory: Sith post TPM have amped powers
So I've been musing at this for a while and now I do believe it is confirmed after watching the Mortis trilogy. Dark siders were more powerful during the Rise of the Empire and Empire eras than they would have been otherwise and light siders were weaker. This applies to both canon and legends continuities.
"I think it is time we inform the senate that our ability to use the force has diminished." - Mace Windu (AOTC)
Now, as we know, the ability to use the force is directly tied to combat ability. Its why Jedi like Yoda and Anakin are the most powerful users we see in Star Wars. Originally this was thought to only apply to the shroud of the Dark Side impacting Jedi's vision. But the ROTS Junior novelization pretty much states that it does impact combat.
"Strong, this Sith Lord is, Yoda thought as their lightsabers whirled and clashed and whirled again. It should not have been a surprise. With the strength of the dark side growing, the Sith must, logically, have grown stronger, too." - ROTS Junior Novelization
Then we get to Mortis.
"You are growing stronger my son." - The Father (Altar of Mortis)
This shows that the reason the Father can no longer contain the Son is two-fold. One, yes he is getting weaker but also two the son is simply growing in power. The Daughter confirms that just proximity to conflict, fear, and anger amps his power.
"Their conflict is feeding you isn't it?" The Daughter (Altar of Mortis)
"As the balance of this world crumbles, so shall war escalate in your galaxy, as my son has descended into the Dark Side, so have the Sith gained strength."
The world itself is an anchor point to the Force. So it also makes sense that if the entire cosmic force is tilted to the Dark Side and shrouding the Jedi as Yoda says it does by AOTCs that the Sith Lord's combat power is stronger and potentially that the Jedi's power is weaker.
Basically the entire galaxy was a dark side nexus during the Rise of the Empire and likely Empire eras and that should be part of the evaluation of the feats of force users during that time period.