Why the Dark Side is stronger
It feeds off of extreme emotions, that allow the user to access unnaturally advanced and devastating abilities (Force drain, dark side worm-hole) that seem to progress each other.
Then there's ritualistic powers unearthed by the Dathomirian Witches and by ancient Sith Sorcerers that allow for spells that can resurrect, and rituals that can immortalize and greatly empower the sorc.
The light side has nothing but probability selection (the Will of the Force), we always see the uber Sith Lords fail to actualize their terrible plans because their plans fall apart, which has nothing to do with the light side user who achieves victory.
Anakin, Luke, Revan, Mace Windu, and Galen Marek were all known to utilize the dark side to progress the cause of the light side, and were able to get extreme momentary amps in power because of it. Especially Revan, and Luke on DSII.
Yet because they were light side practitioners none of them could fully commit to the dark side, the two prime examples of those who fully committed past the point of insanity were DE Sidious and Vitiate who went above and beyond the full scope of their abilities, and then some.