AncientPower
The Chosen One
Firstly, Luke Skywalker states that if Kyp had turned to the Dark Side he could easily become another Vader or even worse. It is stated right after that, that Kyp's full might was achieved. Kyp was using the focal point of the temple and still couldn't locate it, then Kun's power reinforced him and his attempts were described as feeble by comparison.
Kun needed an entire nucleus of followers-including Kyp- to regain his lost reserves of power and return physically. Even after draining Gantoris, feeding off Kyp's hate and drawing from Streen he still couldn't use his full powers.
Kun was not using the Dark Side nexus, infact his own presence has been credited with Yavin IV's power. He was however using the focal points to retain his identity and resist being pulled into the void. Something Darth Sidious thought would destroy him. Kyp himself had already used the focal point before hand to draw out the Suncrusher. A feat by the way which had already significantly weakened Kun before they stomped Luke.
Noting of course that none of the power involved in taking down Luke was Kun's. Something explicitly stated to be down to Kyp Durron, who got so powerful in that instance due to Kun's dark influence. Something Tionne and Horn both state to be the case.
Kun as a spirit, with all the help he got, still wasn't as strong as he once was in the flesh. That is undeniable given the evidence in texts. One example being Corran Horn whom had personal lengthy experiences with Kun and stated that he still did not have a clue how powerful Kun really was.
Just so you know, Exar Kun himself is stated to be a focal point of Dark Side energy, a very powerful one at that. Only comparable to Palpatine-a Dark Side nexus-in Luke's point of view. This is supported by the fact his saberstaff, armor, holocron and talisman are all very strong with Dark Side energy.