Question

Plant with bright flower and small black seeds

Answer 1

The flowers with bright color tend to attract more insects (for pollination). 
If the seeds were big , then they would not tend to stick on the the body of the the insects and fall midway. So they tend to be small as they would stick to the body of the insect and would be transferred to another plant by the insect.

Answer 2

Azara is the plant which has bright yellow flower and small black seed