Personally I do determine the size of a fandom only by their 'works' visible outside their fandoms. Whether it's games belonging to the franchise or other content. Even if non-animated single fanart is the most common, it counts for the least just like text-only fanfics do. I'm not questioning any quality and merely point out what amounts to more publicity.
That's why I wouldn't call it that big or at least not bigger than the others. It is also imho simply caused by the effect where popularity increases popularity. Not simply because of 'peer pressure' or 'unintentional promotion' (any VN newcomer will of course first read the novels which vndb ranks highly, just how it is for anime and myanimelist &c) which of course also play a part but fans themselves being content, if they don't even make it. This work is after all but one case.
As another example, for how quality of something fan made and popularity does not mean visibility, would be that one Skyrim x Zelda mod which is, in terms of quality itself, leagues above its #2349/#2636 place (of around 70000, in the top 5% but will never be found by browsing unless searched for) and a labor of love which can be said to be one of the best Skyrim mods in terms of quantity*quality (countless items, dungeons, enemies, spells) without even alienating non-Zelda fans. It wasn't 'late to the party' either, it is just caused by what is the bane of 'fan-made cross-works'. The boon of course is more publicity.
