Increase size of ceiling joist compromise roof strength?
You don't have joists or rafters. You have engineered roof trusses. The bottom chords are 2x4 because that's all that's required for your scenario. They don't span the ceiling themselves. They're part of a rigid structure that's supported by both the top chords and the diagonal truss members. This is typical and has been standard practice since the 1970s with virtually no static failures on record.

Some trusses, in the case of very large spans or girders (which are sometimes doubled or tripled), are designed with 2x6 or 2x8 bottom chords. But for common trusses in modestly-sized homes 2x4 is standard.

The blocking was probably added to support a particular ceiling finish--tiles or wood panels, for example. Unless you have a particular need for more weight-bearing capacity, leave things alone. If you do, consult a licensed engineer who will examine things much more closely than we're able to from one shady photo.