If you neck a 9 x 19 mm down to take a .32 bullet, you're probably going to be duplicating the performance of the .30 Luger (AKA: 7.65 x 21 mm Parabellum). The .30 Luger case is a couple millimeters longer, but it's 9.9 mm wide at the base of the case just forward of the rim. Just like the famous 9mm Parabellum.
It's not a bad round. A 90 grain bullet moving at 1200 f.p.s. from a full sized pistol.
But, you could load a 90-grain bullet (maybe made of non-lead alloys) into a regular 9 x 19 case and give it a larger gunpowder charge than you could use with a 115 or 125 gr. slug in front of that powder.
Perhaps it's not possible to have good velocity and energy in a necked-down pistol caliber that is short enough to fit in, and feed through, a pocket pistol.
The Japanese had a "baby Nambu" pistol before WWII, and it fired a 7mm diameter, 56 gr. bullet through a 3.5" barrel and a measly 800 f.p.s. Not much stopping power there. That's like a .25 acp.
http://www.yankeegunnuts.com/2010/09/28/bottlenecked-handgun-cartridges/
Interesting quotes at the top and bottom of the page
Why use a bottlenecked cartridge at all? Because you can put more energy behind the bullet, in the form of velocity. At the time, bullets were all round-nose jacketed designs, so expansion wasn’t the goal. Also, feeding reliability is improved, since the little bullet has a big hole to get seated in. Take a look at the specifications on the common early bottlenecked cartridges:
When it comes to handguns, I am not an advocate of heavy & slow bullets (they don’t expand). Nor do I put my faith in light & fast bullets (they don’t penetrate). I prefer medium-weight bullets (within a particular caliber) moving fast enough to expand, and I would sacrifice some caliber or bullet weight to get extra velocity. Given the choice of a .32acp carry gun or a .380, I would carry the .32.