As Colibri mentioned, the modern, smaller critters did not evolve from the larger megafaunal critters. This is true pretty much across the board (I won’t say 100% because I don’t know of every lineage out there, but I’d bet it’s close). Evolutionary trends tend to be one-way: either the lineage tends towards gigantism or dwarfism, but I know of no instances in which a given lineage exhibited both extremes (putting aside, of course, the obvious difficulties in identifying a true lineage from the fossil record in the first place).
Going back through time to earlier ancestors, most lineages start out fairly small in size. Even the first dinosaurs were small (e.g., Eoraptor); some lineages reached truly gargantuan size, but others remained fairly small (and still others remained small and learned to fly…). The first mammals were likewise small, rat-sized critters.
As such, the most likely answer to the OP’s question will be a lineage that started small and simply got smaller. The aforementioned hummingbirds, shrews, and bats are probably the best candidates.