alex / id:
My memory may be fading, but the 737 (-500/400) I've flown had RWY width limitation of 30 meters and certainly were used that way, JAR OPS1 operator. Nowhere I had seen 45m number. The 320, on the other hand, has 45m NOMINAL width requirement, in FCOM 2 you'll find chapter operation on narrow runways and observing that (and certain paperwork) you can go to 30m as well.
link
I suppose the 45m 737 requirement was for the NG, that has noticeably larger wingspan, perhaps?