Nikkor 200-500 VR/TC 14 III @550mm, f8, ISO 360; Nikon D750
Despite what the idealistic tendency would make us believe, the forest niches are pretty tough places to live in.
The size of these tiny birds is an example of one of many adaptations the creatures adopted to successfully occupy a niche (in this two-way interaction).
There is a parallel and actual case when the body size tends to get smaller and smaller under extreme environmental pressures today.
The effects it can have on species can be rapid, from the evolutional perspective almost immediate, and not dissimilar to those that took place in the past (Chicxulub asteroid impact).