Picking them up and noticing which one is heavier works fine.
I've just seen plenty of chickens that were "bigger" because they were taller and fluffier, but much lighter in weight, so I wanted to be sure you weren't making that mistake.
You could try dispatching them at a MUCH younger age-- maybe 8 weeks or so.
People usually want more meat per bird, but that's partly because of the time required to process them. You're doing it really quickly, so you might still have a reasonable meat-for-labor rate if they were smaller. And...