Why do I have smaller than expected floor joist components?
Those are modern engineered floor trusses, or truss joists. They often have flat 2x3 or 2x4 top and bottom chords and a web of diagonals and verticals between.

The 2x you see is a stiffener, or "stiffback", which ties the trusses together to spread load and minimize movement and vibration. 

It looks a bit like someone cut out some of the stiffener, maybe to get a duct in the space below. The builder may have added additional stiffeners nearby.