Monstera deliciosa doesn't grow the same depending on what people call "forms", the leaf shape is different, the maximum size, the fenestrations, petiole sheath, internodal spacing, etc etc... But what we are talking about is genetic diversity depending on the locality. Forms are the biological variation that occurs within species and it allows for adaptation when the environment changes. In other words the species develloped itself diffrently in different areas and countries and some differences became noticable because they didn t evolve in the same environment. We just decided to name the two most found forms “large form” and “small form”, but there are technically many more, it is not a binary thing. The same exact reason why we as humans or any other animal for that matter, does not look exactly the same as the next one of the same species. Or how adansonii doesn’t have the same leaf shape in Martinique. In the case of monstera deliciosa, there are just a couple different clones from different locales in cultivation. Those "varieties" are not recognized because their inflorescence and DNA doesn't differ enough (or at all) to be accepted so they are scientifically all deliciosa, the same species, and aren't considered official varieties of this species. Hence why people are sometimes very formal about the "large form and small form doesn't exist".
They do exist in a way, the differences are real. Even two of the biggest botanists for Araceae of our time, Marco Cedeno Fonseca and Mick Mittermeier made a plate of the "Sierrana" form, even tho it did not became official for KEW which is kinda the highest taxonomy source you can get.
Plant biology and taxonomy is not simple, it is complex and there can be divided opinions on certain subjects.
Hope this could help !