You want to know why Revan is more powerful and what should have happend at the end of Kotor II? They are both pretty much related, although the end of Kotor II wouldn't be the only reason Revan is stronger imo...
Anyways at the end of Kotor II you are supposed to find out you and Nihilus are the same, you can still figure that out if you listen to what is said, but you never actually hear them say it. Their are audio files where you are told however.
Anyways have you ever been at that cave in Korriban. It shows the life of the Exile. From being recruited to fighting in the Mandelorian wars, and then all of a sudden you are fighting Kreia with your friends or against your friends with Kreia. Now this is what the game decides.
If the Exile is dark and you made Revan light then Revan will come up to save the Exile his companions with the help of Canderous HK and T3 he will kill the exile and Kreia.
If the exile is light and Revan is dark then the Exile will show up after Kreia is dead, kill all your friends except for Canderous HK and T3 who will not fight btw and then proceed to kill the Exile...
If you are both Dark revan still owns you, i'm not sure if he kills you or not however, i have not found what happeneds if your both light. Anyways Obsidian first intented for Revan to show up and beat the crap out of the Exile... Giving the Nihilus thing and the Exile meaning all of a sudden. They had to cut it out, because it wasn't finished yet. Still there are aspects that point to this happening in the game if you play through it the right way.