mansour, even though he is 38, is a good one. Luis Ortiz is another one. Had quite an amateur career. Francesco Pianeta, another one, but, he ain't nothing great. Thompson, another one, pretty much at the end of his career.
Thompson, Chagaev, Gomez, Pianeta, Tarver, Harrison, Ortiz (all in boxrec's top 50) Madison, Fields, Perez and Dettweiler are also in the boxrec top 100 (11% of the top 100 then)
Prolly cause guys that size who can fight, are not as prevalent as smaller men who can fight, simply cause they dont have to fight as much, due to the intimidating of their size. Now take that notion, apply it to the fact irrespective of weight, that left handed people are a minority in general, and I reckon that might be somewhat close to the truth.
There's been quite a few recently. Ibragimov and Chagaev helt world titles, CW champ Gomez and Thompson challenged the Klitschkos and there are others such as Ortiz, Pianeta, Perez, the Commentator... and then there's Fraudley. The Beijing Super Heavy final was an all-southpaw battle BTW between Cammarelle and Zhilei.