Just thought I'd chime in a little bit. I am a rather large gentlemen, 6'1" and 320+lbs. I regularly dive to the deeper end of recreational limits, and am starting to touch into decompression diving (for those of you who are just beginning your diving adventure, deco diving is the intentional saturation of your tissue with nitrogen beyond "recreational limits" for the purpose of going deeper or staying longer at depth. To compensate, one makes long stops in the water at various depths to allow thier body to expel the offgassing nitrogen). I usually have no problem at all with any of the diving that I do, including kicking through a strong current, but I will confess that I can get quite winded doing so. Most heavier people have much larger quads and calf muscles to carry thier weight on a regular bases. (If you put these mega muscles on those lanky folk, they would fly like the wind!). The problem with these muscles, is they need more food (Oxygen), and therefore produce more waste (Carbon dioxide). What am I getting at? Well, here's the thing.... that nagging feeling that you get when you are out of breath, or when you are trying to hold your breath, that "need to gasp" feeling is triggered by CO2 in your body. So, it would make sense that if you find yourself out of breath for any reason, that you are producing high amounts of CO2, which I feel, being a large diver, is my #1 nemesis. I try my best to mitigate this by making sure I take DEEP, DEEP, DEEP (did I mention deep?) breaths. This does 2 things for me, it makes me concentrate on my breathing to get it (and my heartrate) under control, and more importantly, it clears out the "dead space" in the breathing passageway. I'm not going to go into the finer points of "dead space" as I believe it comprises almost a whole chapter in the PADI manual, but in short its the distance between the air supply, and your lungs. If you do not take a large breath, a larger percentage of that breath will be part of the last one which contains... you guessed it CO2. One more tip, when you complete you OW class, most organizations out there have a bouyancy training class that you can take. PADI's is called "peak bouyancy". I would advise taking this from a good instructor, as it will teach you proper trim and bouyancy control through breathing (I think someone mentioned this earlier in the thread). This is extremely helpful to larger divers, because we need to try and be as streamlined as possible to appear smaller and less threatening to the sharks (so we don't scare them away!).
BTW, the observations about thermal comfort are true in my case as well. I routinely dive Ginnie Springs (72d) in a 3mil Henderson shorty, and I am comfortable even after a third consecutive dive).
Anyway, I hope my babbling helps in some form or fashion, and I wish all divers, mass challenged and lightweights alike, safe and enjoyable diving!!!
