Kessler is not that big for the weight. In fact, he definitely looks a bit less physically imposing these days to me than he did pre-Calzaghe (and I'm not just saying that to big up Joe either. I'm a huge Kessler fan too), well definitely since he had that layoff prior to facing Pedermo if not then. I suspect all the injury problems he's suffered throughout his career (and he's had a lot of them) might have have necessitated a cut down in strength work in the gym. Check out this excerpt from a Danish documentary I have about him where his strength and conditioning coach talks about on how fragile his body is and how it affects his motivation in training. This was filmed during the build up to the Beyer fight in 2006 too.
He weighed in bang on the division limit of 168 for Calzaghe, as he had done on all but one occasion for his previous eight fights before that too. In fact, he was even slightly over the first time of asking. Calzaghe badly ****ed the weight up for that fight too, coming in at the joint lightest of his career at 166½. He looked like death warmed up on the scales. However, with the exception of the Sartison, Pedermo and Green fights (the later of which was at light-heavy), Kessler came in bang on 167. He was even 166¾ for the Bouadla fight. I know Andre Ward was very surprised at Kessler's lack of size when he met him in the flesh for the first time too. He said he'd expected him to be much bigger than he was.
Like Kessler, Froch isn't particularly big for the division either. I know he was only 170 for the Abraham fight and it wouldn't surprise me if Kessler's average weight was about 176-178 max come fight time. Remember that in this day and age of drastic weight cutting many fighters (particularly those from across the pond) cut huge amounts of weight and come in a good deal heavier after rehydrating. Some of them put on in excess of 20 plus pounds for god's sake.
Fighters like Kessler and Froch could fight down at 160 if they really wanted too, perhaps even 154 if they were willing to put their bodies through the kind of hell severe weight cutters like Gerald McClellan, who was a ****ing cruiser come fight time, do. Victor Ortiz was roughly the same weight as Froch against Abraham when he fought Floyd down at 147. Jimmy Montoya (Kessler's trainer) said that Kessler would go down to middleweight for the right fight too.
Even during his brief flirtation with the light-heavyweight division against Allan Green he only weighed in a pound and a half over the division limit. That fight was actually a catch weight bout of 172 and I thought it was stupid of him to afford Green (who is massive for the weight) the huge luxury of not having to sweat off those extra few pounds. I think both Froch and Kessler would be undersized up at light-heavy and would run the risk of getting knocked senseless by some of the boiled down cruisers who ply their trade up there. That said, the division is pretty weak at the moment though.