Ah, but you don't have to have them the exact missile sizes.
You can have a size 1.02 missile launch a size 1 missile.
And the 1.02 would have been launched by a 1.04, which had been launched by a 1.06, and so on and so forth.
The size 2 would have nothing but engines, fuel, and one of the size 1.2 or so missiles. The 1.2 would release a 1.18, releasing a 1.16, releasing a 1.14, releasing a 1.12, releasing a 1.1, releasing a 1.08, releasing a 1.06, releasing a 1.04, releasing a 1.02, finally releasing the original size 1.
That would just be for a ball of chaff to take up anti-missile fire though, to actually get them all the same speed as the end product warhead would require more size into each missile. Still doable, though. For instance, I just made a size 10 missile which releases a size 8.6538 missile which releases a size 7.8054 missile which releases a size 7.03 missile with a size 36 warhead. All go at 12,500 km/s, have .1 active sensors, and the last one has enough maneuverability to have a 75% hit rate against a target moving 10,000 km/s. I'm sure it could be optimized quite a bit (removing the sensors from all but the final one would help cut size down, and still confuse AI AMMs).
I tested them by making two races and having one shoot the other. They showed up on sensors first as size 10 missiles, as they got closer the sensor detected the size 9s, then size 8s, then size 7s. It didn't work though, because auto PD for some reason destroyed all the size 10s then all the size 7s then the 9s then the 8s, which meant it killed the warhead missiles before most of the duds.
The swarm of duds would still be a viable tactic though. With just a handful of missile launches you could have a few hundred missiles heading at a planet, making them fire hundreds of AMMs to destroy them. Once they've used up their anti-missiles you could switch and fire the real ones.