You can see
HERE how surly actually measure their LHT frame, and that the seat tube and top tube are basically the same. What makes it an apparently 'big' frame is the combination of lower BB height + relaxed gometry + long chainstays which add up to quite a long wheelbase.
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"Surely one can love his own country without becoming hopelessly lost in an all-consuming flame of narrow-minded nationalism" - Fred Birchmore