I am... suddenly realizing we're looking into the matter from the wrong end, as it were. It's not fierce love for someone that gives you power (as displayed, at least), it's love from someone. I.e. Chrysalis has no love to give whatsoever, yet utilizes it to the fullest. Celestia, even if she loves her subjects, would require them all to actually love her in return, which I don't really see happening. At most, she has her sister's love, plus whoever is in her "closest circle". From what we've seen of ponies' reactions to her, she's revered as an old, wise, and powerful Princess, in the "matriarch ruler of all the land" sense. Very few would see her as any more than a reasonable authority figure.

Or, I can bring one of my favorite modern-fantasy universe mechanics into this, and liken the Power of Love to, you're going to laugh, the Twilight (or perhaps the Gloom, to avoid the likeness), from the Night Watch series by Sergei Luk'yanenko. All people exist in a balance, between the amount of magical energy they draw from the Gloom, and release into it. Completely normal people release more energy than they draw, but the most powerful mages draw more than they release, acting as "sinkholes" for the power. If the Power of Love works in a similar way in Equestria, it may be that Chrysalis could beat Celestia because for all the Love she draws, she releases none converting it all into raw power, whereas Celestia is in balance, but has more normal power through drawing it raw, from her Sun.