After looking over at all my pots, I would say I agree with BigBella...that it's cultivation. If plants in different-sized pots are well-established, they usually form similar-sized traps. At the moment, all the VFTs in my shallow pots have large traps while only one of my deeper-potted VFTs have the large traps. I attribute this to simply because the latter had been in its pot for a couple of years now while the other deep-potted VFTs had been growing in theirs for only a few months.
This also goes along with something else. The ones that grew the largest for me are also growing in older peat (darker, less fluffy) that's more tightly compact. VFTs in fresh peat that's looser tend to send out smaller traps and be less vigorous in general.
Carnivorous plant enthusiast