We own 2 puddle jumpers...bought for my (then) 3 y/o and just 2 y/o...for pool use (just to make handling 4 kids in the water easier...and this year, 5). My 2 y/o (will be 3 in July) has tremendous mobility in it. It is pulled all the way tight on her back, and it stays where it's supposed to stay. It fits her shockingly well, and has for the last year (since we got it...I guess more like 9mos?). She is a little over 35" tall and weighs about 25lbs, with an 90%+ HC (all my kids have giant noggins), to give you an idea of her size. She's been 23-25lbs for the last year or so, so she's not radically bigger now than she was then. The puddle jumper, if she's not careful, can still force her onto her back (and she has difficulty rolling back over so she can more actively swim). If she's relatively vertical, and does something to lose her balance (so to speak) and she falls, the default is to her back.
My ds is 8mos, weighs about 18lbs and has an 18" chest circumference. I have not yet tried the puddle jumper on him, but I could, see how it fits.
My now 4 y/o wore it, this year, over a wet suit, and I had to loosen it off the tightest it could go in order to accommodate the wet suit (she needs it for thermal regulation). I think the wet suit limited her mobility more than the puddle jumper!