I can imagine that in a 9mm cartridge, the bullet plus the powder take up the full space. If the bullet is larger, then the powder is smaller and vice versa. With less powder, less felt recoil? Is that the idea?
This is not my area of expertise but here's what I did - My brain tries to tell me a heavier bullet should have more recoil (for every action there's and equal and opposite reaction). It stands to reason it would take more force to push a heavier bullet. But, as jb pointed out, not everything is equal. Typically the heavier bullet is not being accelerated to the same speed.
I thought if I just looked at a measurement of force, maybe this would make some sense. Foot-pounds is a pretty easy measurement of force to calculate and some ammo manufacturers publish it. Using Freedom Munitions published numbers for round nose 9mm (weight and speed) I cheated and used an online calculator to get ft-lb
I believe this explains why heaver rounds have less felt recoil. I don't know if that necessarily means less powder was used (re-loaders would know more about that) BUT the resulting measurement of force is less.
If all things were equal - enough explosive force was used to make every bullet travel at 1120fps then:
124gr would be 345.4 ft-lb
147gr would be 409.4 ft-lb
In which case there would be significantly more felt recoil.
Yes, the boxes that I'm looking at make clear that the heavier bullet is slower (a little slower) than the lighter ones. Since I'm looking at Hornady self defense, I assume that there is enough power and penetration to be valid self defense charges.
The conclusion is the same as you'll find most everywhere today - handgun rounds are not great at "stopping" (any handgun round). The most important thing is shot placement and the understanding that multiple shots will be necessary. With that in mind, low recoil becomes desirable as long as the ammo is still performing to acceptable standards. From what I've read, this was a big part of the FBI's decision to switch back to the 9mm round.