I really don't see the problem. Robert's a fat, jovial, ineffectual king. The days of badassery are rather distant memories, with Renly much more like the Robert that defeated Rhaegar than the Baratheon who sits the throne. He SHOULDN'T seem like someone who is capable of toppling a kingdom and taking it as his own. In a sense, he never was: I always got the impression that most of the strategy and political maneuvering was done by Eddard and/or John Arryn. Robert took the limelight so that Eddard wouldn't have to.