I've used Fat Beams a lot on my main basses over the years. The sets I've used always included a .125 B string. Something to note... DR explains a bit of their construction process on the back of their packaging. They add up the layers of windings for you, which shows they use more metal in their strings, then compress the string down to equal its advertised guage. For example, the layers of windings in a .125 B string add to .128, then they roll the string to compress it, and end up with a .125 string. This does, in my experience, add some stability when down tuning, but the Beams series from DR are still Round Core strings. Not as stiff at tuned tension as a hex core string. A .120 B prolly has .124 or so worth of metal. I don't know the exact numbers, as I've never used a B string less than .125.
The Fatbeams are my 2nd choice overall in strings. I like the feel and tone, and the B string hangs with the other strings quite well. But I don't like the .125 B their standard set comes with. I prefer .130, for the equal response of the B vs the other strings. For me, a .120 B would be too small. But, DR make quality strings, and if someone is accustomed to the smaller B from other sets, they may well be very happy with the performance of DR strings overall, and Fatbeams specifically. And, if true about the "more metal", and not just marketing hype, a .120 B is nearly a .125 with the extra mass.
If I had to use a .120 string for my B, DR would be my choice. If a round core B in any guage isn't "stiff" enuff, try a .120 B in a Riders series from DR. Low Riders are Hex core, which stiffen a string slightly and can be great if you want a more resistant string in a .120 guage.