Because I'm 63 and no longer like to carry a large backpack type carry on and I only do tropical. I use a 22" wheeled carry on, by Eagle Creek which is also great for any non dive flights as well. Also because I am slender, I can fit everything but fins and camera . I can fit my Dive Right Travel Pac BCD w/ sausage and snorkel, my regs and pressure gage , my 3.5 mm wetsuit , beanie and booties , mask, wrist and BUD computer, DRIS shorty dive light, one change of sox's and un's , fishing shirt and zip leg pants/shorts. small travel toothpaste and brush ,floss and extra reading glasses.
Then on my next trip I will also carry on a medium size camera bag With Olympus EMP 1 and housing and Sola video/focus light and small Ultralight arm system.
The only thing I cannot fit in is my Atomic split fins they go in the checked bag with the rest of my clothes and boat bag. knife etc.
However I am looking into some Hollis F2 fins to see if they fit, If they do It will be a tight squeeze for height, the Eagle has good compression straps, so we will see. Cheers , Kev