Nitrogen has a smaller size than Bismuth, because on going down the group the atomic size increases i.e. N<P<As<Sb<Bi, as shown below:

Due to smaller size, there is very high electron density around Nitrogen as compared to Bismuth. Therefore, nitrogen atom can easily release electrons. And, we know higher the electron donating tendency, higher is the basic strength. Due to which NH3 is more basic than BiH₃.

Note: The basic strength in group 15 is in the following pattern:

NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃