Why is there such a big difference between LED brightness?
Possible reasons

You seem to be powering the LEDs directly from a battery. Different LEDs have different resistance and different forward voltage drops. So, when you power directly from a battery different LEDs can draw different amount of current / power and output brightness is dependent on the input power.
The efficiency with which different colour LEDs convert input power to output light is different since the materials used and construction are different.
Our eye / camera sensor are sensitive to different colours at different amounts. (But blue would have had lower brightness then, I would think).