There are many reasons why two birds of the same species are differant sizes. In many species males and females are differant sizes. Sometimes that can be easly seen and sometimes it is only a few milmeters, but it can be measured.
The two birds in these pictures are juveniles. The top one is a juvenile male and the second picture is most likely a juvenile female (it could be a juvenile male). My first question would be, what is the age differance between the two birds. One bird could be several weeks older than the other bird.
Rayfield Pye