MiniVend is not just a script. It is a combination of many programs, Perl modules, and links to other subsystems such as SQL databases, CyberCash, PGP, and the Glimpse search engine.
It is so big because it is more than just a shopping cart -- it is a complete database access and retrieval application. It uses a lot of memory because the author got a case of feeping creaturitis and added too many features while attempting to maintain backward compatibility with older catalogs.
This means that MiniVend is probably not a good choice for someone who is going to offer only a few products. It is optimized for catalogs of more than a hundred items -- catalogs that expect to change and grow over time. It is suitable for very large catalogs.