http://www.arniesairsoft.co.uk/forum...howtopic=65787
essentially it is the size; g23f is 198ish mm while g18 is 210mm (goin by memory here)
also note that the g23f has a hybrid barrel, as oppose to the ported(or is the correct term compensated) barrel on the g18c
both work the same way, and as with all ksc glocks, smaller guns can use bigger mags but not vice-versa.
edit: correction, the g23f is 185mm, g18- 202mm