I presume this is a hypothetical question (not can a 200 pund man squeeze into a xxs wetsuit question). In recent years wetsuit flexibility is the field where biggest improvements were achieved. Neoprene itself is extremely flexible, it is the jersey material used to line the neoprene that is/was the problem. Wetsuits weren’t stretchy because the nylon couldn’t stretch enough. Today the materials used to line a neoprene are lighter, stretchier and silkier than ever.
So how much can a wetsuit/neoprene stretch? The top of the line stretchy materials used in wetsuits can stretch up to five to six times their original length. Those cheaper ones can stretch like three to four times.