Second Peak in X-Ray Diffraction Pattern of bcc Metal
The reflection with $n=2,\;(h,k,l)=(1,0,0)$ is the same as $n=1,(h,k,l)=(2,0,0)$. Yes, I said "same", not "similar"; they are one thing, there are no "both". For this reason, you may just as well abandon using $n$ altogether.

As for why the peak is so small compared to others you've seen, let me respectfully suggest that maybe you haven't seen all that many. Indeed, there is no law that all peaks on the diffraction pattern must be about the same size. On the contrary, they are essentially proportional to the squares of Fourier coefficients of electron density, and as such, can be anything from 0 to the maximum. The early textbook exercises usually employ some mercifully simple cubic or tetragonal structures with few well-resolved and well-developed peaks. Not all compounds are like that. Look at the graphite diffraction pattern, for instance: you'll basically see a lonely (002) peak, large as life, and only after the most careful inspection you'll start noticing the rest of them amidst the background noise.

As for the (100) peak, it couldn't be there, since it fell victim to the systematic absences of the bcc lattice. Ditto for (111) and (210). Only those with even $h+k+l$ survived, and that's precisely what you see on the picture.