So the terms are coming from the Buddhist tradition, where there is a category of Buddhist denominations usually (a) valuing the Lotus Sutra (as sort of a central uniting point and/or shibboleth); (b) believing in celestial buddhas and bodhisattvas who you venerate and whose assistance you request in life; (c) valuing the idea of us all being "on the path to nirvana" (the "bodhisattva" ideal) even if we are not monks; and (d) representing the diversity of Buddhist denominations as largely being "lesser vehicles" (hinayana) to nirvana, and their own category as being a "greater vehicle" (mahayana) to get there. "Maha-" is a prefix meaning "big," but often also connoting "great, impressive, respectable, noble".

In practical terms a "big-vehicle Prelude" is saying "I want to do something much smarter than the original Prelude and I am willing to replace most of the underlying abstractions in the Prelude to do it," which can sometimes mess with code that has to deal with old-prelude and new-prelude subsystems of code. A "small-vehicle Prelude" is much more "I have a couple extra modules here and there that I want to import, and maybe I want to fix a couple of these operators to a slightly more general definition with a typeclass, but largely I want everything to be the same, hence intercompatible with minimum fuss."