The phrase "smaller litters" refers to a situation where a group of animals, particularly mammals, gives birth to fewer babies at once compared to what is typical for their species. Full definition
Small litter box - Maybe the litter box you purchased for your kitten was a perfect size, but now it may be too small for your adult cat to comfortably eliminate. (blog.petmeds.com)
However, obesity during pregnancy can result in smaller litter sizes and death of kits at birth. (articles.extension.org)
Like other sharks and large rays, whale sharks likely have small litters of pups and are slow to reach sexual maturity. (sciencedaily.com)