The 3DS is closer in dimensions to the DSl than the DSi, which was slightly larger but thinner. So a few millimeters and grams here and there will hardly make a significant difference.
If they did anything to the small form factor 3DS, it would maybe be to the ergonomics (less blocky, no hard edges) and maybe make it a bit thinner, but unless that could be done in a cheaper manner from a production standpoint, there may not be much point in a redesign other than as a product refresh to reinvigorate sales.
Naturally, it's the common "I'll wait for the inevitable 3DSl" comment that Nintendo wants to head off at the pass with this statement.
If anything, and consumers stay hyped on the uniqueness of the 3D feature, Nintendo should and probably will release a larger 3DS with a significantly larger 3D display, which would be a better product, likely with better margins.
But, pending consumer demand, and Nintendo's ability (or lack of) to match production, that's probably a good two years away from initial launch.
