Let's say we have network 126.96.36.199/16.
Shorter prefixs than /16 are /0-/15. How about smaller prefix? The term "smaller prefix" is found in BGP-related documents.
I would assume /17 to /32 are the smaller prefixes.
Prefix is used as a synonym (1) for network in this case. The prefixes are longer, thus the networks they describe are smaller. I can see this could indeed be considered confusing, smaller as the opposite of shorter, but that's natural languages for you.
(1) It's probably a metonym, but that's a discussion for a different stack exchange site...