A quick check of the Sterns website shows that the "33 gram inflatables"
offer 33 lbs flotation. The 24 gram units offer about 22 lbs.
Keep in mind that you are not standing out of the water, standing on the PFD, but rather most of you is ion the water ( nearly ) floating. It does not take much to keep the head above water, and/or rotate the torso so that the mouth of the wearer is above the water line.
Trust me. 33 lbs flotation makes a big guy float like a cork.
If concerned, put on a vest and jump overboard ( in protected waters, etc... ). It is a good safety exercise and will prove the point nicely. If you -really- want to test it, do this in a pool, as fresh water provides less floatation than brackish/salt.
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In fact, all who have an inflatable really, really should test them out. The auto-inflate types will scare the sheep out of you if unprepared. Best to try one out under controlled, safe conditions prior to exploring new product usage while all else is falling apart.
A good time to do this is end-of-year ( or first of year ) vest maintenance. Don it, jump in, float around a bit then carefully recharge according to mfgr instructions after drying it out.