I’d like to know this too. My guess would be that the overall average is lower for free users. Not because a large list indicates a “power user”, but because users who don’t use their watchlists (e.g. those who don’t know how, don’t see the need, use alternatives, are inactive, etc) are less likely to be VIP members. All those unused lists will push the free tier average down.
And yet, the free tier also includes many users who simply can’t afford every streaming service or theatrical movie release. Having limited access to content means their watchlists are likely to fill up with currently unavailable items. Those free users may well have a higher average watchlist size than VIP users, regardless of how much they use Trakt. They’ll be among those most hurt by the recent changes, since they most need larger lists, but can least afford to pay for VIP.
Even for VIP members, a large watchlist doesn’t necessarily signal increased usage. If they’re paying for VIP, they presumably have access to all the content they want (why pay to watchlist a show if you can pay to watch the show instead?), so list size probably depends mostly on how much free time they have versus how much they want to watch. But unlike the fixed 100 item limit, time is relative 
- Completing a 100 item watchlist within a week is a lot
- Completing a 100 item watchlist within a decade is not
Continuing the discussion from Freemium Experience: More Features for All:
Someone who only has time to watch a few things per year isn’t likely to be using Trakt daily. When they do use it, they’re more likely to add some new movie or show to their list than to check something off. Their watchlist will keep growing slowly over time, with its size being proportional to how long they’ve been using Trakt. With such limited use, they probably won’t be using any advanced features. This doesn’t match the “power user” concept as defined, and yet they’ll eventually reach 100 items and be forced towards VIP.
Conversely, a user with lots of free time will find it easier to keep their list small. As a true daily power user, they might be checking things off just as fast as they add them. The list may stay below 100 items, but those items will be changing constantly. They might also be more likely to use features like scrobbling, calendars, and various apps to keep up with the increased use. Doesn’t all this put more strain on Trakt’s systems than storing a few extra rows in a database? Why is this considered “casual” use?
I’d be curious to know the metrics, but I suspect daily usage correlates more with a high rate of watchlist modification than with a large watchlist size. If the aim is to set a “threshold from casual to power user” based purely on list usage, I would think the watchlist should be rate limited, or capped to a size proportional to years of membership (as suggested by @mbirth and @GHax1111), rather than capped to a static 100 items. 
Personally, I appreciate the Traktiversary additional lists (I mainly use lists to work around certain Trakt limitations, such as tracking partially watched movies/series or where details are uncertain, and the new limits made a couple of my existing lists unusable), but the drastically limited lists are a problem for too many users. The 250-500 item limits proposed in 2022 were already worrying; 100 items is outright painful, especially for those of us already over the limit. It’s ruining the user experience for people who can’t/won’t upgrade to VIP under these circumstances, and is having a knock-on effect on others, both existing and potential users.
Sorry this turned into an essay. 
TL;DR: The 100 item limits are hitting people who aren’t power users. Not good for Trakt or its users.