the bullet usually performs some permanent expansion (plastic deformation). So that is the bigger reason why force is always higher the first time bullets are seated.
Yes it takes more & more work to reform brass with ever bigger dimensional changes, but this does not change the spring back force gripping your bullets. The moment you stop adding all that energy, the neck just falls back into it's normal balance.
Test it for yourself:
Neck down
1thou (after spring back), seat a bullet, pull the bullet,, the neck springs back inward ~1/2thou.
Neck down
5thou (after spring back), seat a bullet, pull the bullet,, the neck springs back inward ~1/2thou.
Neck down
10thou (after spring back), seat a bullet, pull the bullet,, the neck springs back inward ~1/2thou.
So any way, the only attribute holding a bullet is the Force X Area of that ~1/2thou spring back.
If you think a bullet is held back (ballistically) by friction, you're wrong there too.
Test this for yourself:
Size necks all the same, take half to squeaky clean with a bronze brush, dry lube the other half with graphite or tungsten. Note while bullet seating that forces are way higher with clean necks (higher friction).
Shoot them across a chrono, and you'll see no difference in MV.
So bullets were neck released before overcoming frictional difference.
Now, if you want to see tension change, and MV change with it, simply adjust the LENGTH of a given neck sizing, to grip more or less seated bullet bearing. ---Sprinback Force X Area---