Explain why $NH _{3 }$ is basic while $BiH _{3 }$ is only feebly basic?

The atomic size of nitrogen is less than that of bismuth. As a result, electron density of the nitrogen atom is greater than that of bismuth atom. It means that electron releasing tendency of ammonia is more than that of $BiH _{3 }$. That is the reason, ammonia is more basic than $BiH _{3 }$ which is only feebly basic.