The only body builders who have to be concerned about PEDs or steroids being detected are those who compete in events specifically for 'natural' bodybuilders so I wouldn't have assumed that a bodybuilder would definitely know a GREAT deal about the detection of PEDs.

To answer your question, Pac won't necessarily look any smaller...

If you take juice but don't train or feed yourself to get bigger then you won't. Try to think more of PEDs and steroids than that they just make you 'bigger'.

performance enhancing drugs.

Like you've said, he may fatigue later on if he has used them and stopped, lose some of his explosiveness etc.