I have two theories about how his strength works.

First one is that he has his hugh chakara as his own, and the chakara he gets in battle slowly dissipate over time since it's not his own. If that is true the differences are huge between him and Zabuza.

Second one is that he might actually have been on the same level as Zabuza to begin with but he actually keeps the absorbed chakara. And every fight he ends up with a bigger chakara-pool. Because a enemy must make Kisame spend more chakara then the enemy himself has for him not not end up gaining chakara. And logic says that the victor should be the one spending the least amount in the majority of fights. And since he absorbs chakara from his enemy, with every hit the enemy loses strength which reduces his chance of inflicting damage while Kisame's strength increases which increases his chance of avoiding and reduce the damage. So though time he just kept getting stronger until he was considered a monster.

If the second one is the one that is true or close to it then he and Zabuza where probably almost at equal strengths at one time. But after all fight while being with his village, and then after he became a missing-nin and got to fight everyone hunting him. ANBU, regular mist ninjas that cross his path, rival village ninjas that has him in their bingobook, his own targets that he hunt. And then he joined Akatsuki, and made even more enemy's, and he got sent after big targets. After all that time Zabuza should seem like an ant next to Kisame.