The 9mm Glocks slide is almost exactly 1" wide. The Glock grip, the widest part of a Glock is a smidgeon over 1 and 1/8" wide. Got the gun. Got the ruler.
I don't have a P239, but I have handled them, and they seemed pretty slim, but I did not like the way the slide was so much higher over the grip. It seemed top heavy, whereas the slide on the Glock sits nice and low in the hand.
The Glock catalog lists the G19 as 1.18 inches. SIG's lists the P239 as 1.2 inches. That little bit of difference is not worth mentioning.
The G19 is 6.85 inches long, 5.0 inches high, and 1.18 inches wide. It weighs 20.99 oz unloaded.
The P239 is 6.6 inches long, 5.1 inches high and 1.2 inches wide. It weighs 25.2 oz unloaded.
Mox nix. (Makes no difference).
But: G19 post ban mags hold 10 rounds, pre pan 15. Pre or Post, the P239 only holds 8. A difference. For the restriction of rounds, the P239 should be much slimmer.
By the way, the Kahr K9, T9, MK9, PM9's are all .9 inches wide.