Lots of good points brought up here.
I'll give my opinion, based on real life experience and statistics. Bear in mind I am mostly speaking about the fetish community in general, not just tickling.
First of all, according to the CDC online, almost 70% of the adults in the USA are classed as overweight, and from what I have seen the same statistics hold true for the fetish community, both men and women. I have seen the same ratio in TNG, which means The Next Generation and is for kinksters under-35, groups as well as older gatherings. I do believe the percentage of overweight kinksters (and vanillas) is slightly lower in large cities, where people are more likely to have to walk a lot rather than drive everywhere. I have not noticed a greater proportion of overweight people at tickling events as opposed to general fetish events.
Secondly, I would, as others did, ask the OP what he classifies as "heavyset" as it would help me to understand where he is coming from. A lot of men get a skewed perception of normal from watching porn/tickling videos, or attending foot parties, etc, because the majority of those women are chosen to appeal to a mainstream ideal, or fantasy, if you will, which does not reflect what the average woman actually is.
Thirdly, I don't really agree with the comment someone made about more of the smaller women being in relationships and therefore not being "on the shelf"- as Fetlife is not a personals site, but rather a "Facebook of fetish" lots of those posting, and listing fetishes ARE in relationships (raises hand) Just because someone's listed a fetish on there doesn't mean they are looking for a person to do it with.
Fourthly, if the OP is correct about the percentage being a bit higher in tickling than in other fetishes or the vanilla world, there are a couple possible reasons. First of all, if you have bones near the surface of the skin, tickling can be kind of painful unless the ler' is extra careful/uses a light touch. A few bad experiences might put slimmer people off of it. Also, from my personal experience and from anecdotes I've heard- tickling fetishists tend to be more- how shall I say this nicely?- fixated, than other BDSMers. I don't really know why this is. Because of this, if a female tickle fetishist happens to fit this mainstream media standard of attractiveness, she is more likely to lurk, or in the case of Fetlife, not list tickling as a fetish to avoid unwanted or excessive attention. In addition- I have heard people say they are more ticklish after weight loss. They may then find tickling to be too intense.
And last of all- those that enjoy tickling but don't really have any other fetishes and are otherwise vanilla are much less likely to BE on Fetlife than general BDSMers. I would be interested in seeing if the OP's theory holds with examining those into other popular fetishes (say spanking, bondage, etc) and then comparing those results/percentages with, for instance, the contents of an average full subway car.