Grab the suit with your thumbs about an inch apart and pull hard until it won't stretch any more. Your thumbs should now be twice as far apart. You'll probably lose more heat to the wind, between dives, than to the 80-degree water. A torso that sheds and dries quickly will keep you warmer. One advantage of these suits is that they really will fit easily in carry-on luggage.
None is bulky, but the smaller the better.
Weight is a pretty good indication of bulk. Thumb loops and stirrups help you use the suit as a layer under a thicker wetsuit, an unproven but interesting concept. Knee pads and spine pads may add comfort and durability, but also add bulk, weight and cost. Not very important, in my opinion, since even the thickest has very little insulation. Keeping the water out is the name of the game.