Comparison of tailed-beast power levels has not been explicitly stated in manga, or the anime. But following things have been mentioned in the manga:
1) 10-Tails is the origin of all of the chakra.
2) 9-Tails is stronger than all other beasts combined (Of course, not counting 10-Tails).
3) 9-Tails, with only half of it's total strength, was able to counter 5 other tailed-beasts.
Based on those three observations, I have a theory that seems to fit in fairly well with manga.
Since tailed-beasts aren't the only beings with chakra, it is safe to assume that 10-Tails has more chakra than all the other tailed-beasts combined. Also, Hagoromo created 1 to 9-Tails by dividing 10-Tails chakra, and he made 9-Tails stronger than other tailed-beasts. He must have followed the same pattern for all the tailed beasts, unless he randomly chose to make 9-Tails overpowered, which seems very unlikely. So, that leads to my assumption that 8-Tails must be have more chakra that the combined chakra of all tailed-beasts having less tails than 8-Tails, and so on.
Now, using the following classic geometric series:
Let the amount of chakra of a tailed-beast be denoted by 'x/[2^(10-n)]'; where x is all of the chakra, and n is the number of tails of the particular tailed-beast.
Based on that formula:
10-Tails has 'x' amount of chakra, which is all of the chakra, and is consistent with observation 1. 9-Tails has 'x/2' amount of chakra, 8-Tails has 'x/4', and so on. This way, each tailed beast will have slightly more chakra than combined chakra of all tailed-beasts with lesser tails. And that is consistent with observation 2. Also, according to the formula, 1 to 7-Tails' combined chakra would be less than half of 9-Tails' total chakra, and that explains observation 3.