I don't think this is entirely true. My FNS 40 is a little wider at the top of the slide than my FNS 9. My FNS 9 is the same width as my FNS 9c at the top of the slide. I would say 1.25" at the frame and closer to 1.15-1.20" at the top of the slide for the 40.
The 2015 FNH USA Catalog has a list of specifications for all the FNs. It lists all of the FNS variants, both 9 mm and .40 S&W as being 1.35 in. wide. The online spec page for the FNS-40 LS has the width as 1.35 in. (top) and 1.55 in. (bottom.)