I've thought that too actually.
1) the grant and Jerry puppets will always bleat on about how great it is for everyone and their grandmother to lsoe weight, but that's because they were skinnny little runts themselves as youths. For big guys, in any sport, power is as important as being lean. Its better to have both. But forced to choose, you take the power.
2) back during his rookie season there was some article talking about the way Cousins did not do/refused to lift weights. Which is just one of those classically DeMarcus I have the kneejerk stubborneess of a 16yr old things. from the look of his body, its still true. He has alot of natural strength, but nowhere near what he would have if he hit the weights.
3) However, I also think his increasing concern with not getting into foul trouble/into it with the refs, has made him less aggreesive on the offensive boards in particular and the rebounding is just one part of the general decline/discomfort in his overall game thus far. If he were somehow to ever find a groove this season, I would not be surprise to see it bounce back upwards.