I have been wondering about this for a while:
In Bamidbar, 3:43, it mentions that the number of firstborn males (one month and older) was 22,273. Now, we were just told that the number of total males (ages 20+) was just over 600,000. This gives us a ratio of ~1:30 for firstborn:children, which seems rather low.
Now, maybe there was a disproportionate number of female firstborn, or perhaps there was some other qualification that disqualified a large percentage of firstborn from officially being "firstborn", but I haven't ever come across anything suggesting that.
Does anyone know of any source addressing this point?