Bolbitis is a well known genus of old world aquatic ferns with the African species B. huedelottei being by far the dominant species in aquaria. Occasionally B. heterclita is offered as an aquatic but it isn't. In decades of asking for a photo of it growing submersed I have seen only one that did not die underwater, and while it did grow it did so only marginally, making a couple of tiny leaves over the period of a few months. B. heudelotti, once established and grown to be mature, which in practical terms means a large plant with large leaves, will grow rather quickly at that point. The other species are not common but around the end of the 2010s new forms appeared in the hobby and B. difformis is starting to be seen as one of the more attractive forms
Whether there are "small" and "large" forms of this species is in question. A few claim are however Tom Barr asserts all the "small" forms he has tried all turn into the "large" form. Dr. Barr, aquatic botanist by training and vocation is one of the most successful growers of aquarium plants of the century which strongly suggests small "forms" are either immature of poorly fed examples of the species. We await actual evidence that the two forms are in fact distinct and if so here they are, if not then this is what big and small examples of the plant look like.
Family Dryopteridaceae Herter: 26 genera, ca. 2050 spp. [2102-175 species listed]. Family circumscription fide Smith & al. (2006), who transfer Elaphoglossaceae, Bolbitidaceae and a part of the former Lomariopsidaceae into Dryopteridaceae. Taxonomic order of genera as listed here is quite uncertain.
Genus Bolbitis Schott: 60-65 spp. tropics / subtropics, mostly SE-Asia [66 species and 7 hybrids listed]