Well, although I do agree that Bridges knowing how to intuitively use the bigger iron man suit was part of the suspending belief, it would also have made for a bad movie if they showed us his learning curve. His evil was sort of a surprise, it didn't need to be telegraphed like that.
Also, Bridges DIDN'T build the suit, he had all those scientists build it. They just didn't know how to
power it. That's why he stole Stark's chest piece.
...with a special tool designed specifically for the job that fit and worked perfectly.
And as far as him building the suit in a cave while being guarded... I had absolutely no problem with that. He was, after all, building it in pieces, and except for the helmet and chestplate, everything else, arms and legs, looked like missile parts.