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Paul Ryan missed a HUGE opportunity last night during his RNC speech -- by NOT showing off his rock hard physique ... at least according to Congressman Aaron Schock.

We caught up with Rep. Schock as he left an event for youth, sponsored by MSNBC's Chuck Todd, in Tampa yesterday -- and asked if the Republican VP nominee needs a fashion makeover.

As TMZ first reported ... Ryan works out religiously at the House gym, and we're told he's even hiding a six-pack ... just like his fellow Republican Schock.