I have run into similar expansion/penetration with bottleneck calibers. Specifically, 150 partition from 7mauser and 7mag. I think it's the higher velocity causing rapid expansion, like a funny car chute. The 7mauser hits the medium slower, opening the chute only partially, thus penetrating equally. Odd analogy actually.
The 45-70 has a reputation for extreme penetration, it would of course not matter on a plinking/pumpkin load. On game, I would want the bullet to both expand and penetrate. It doesn't help killing performance if the bullet penetrates a 6" Alder after passsing through your animal. Would be easier to match this as nowadays, the 45-70 is usually a 0-150 thick bush cartridge. It's not necessary to worry about performance at 50 yards and 500 yards such as bottleneck magnums.
For fun plinking loads. Me and my cousins are running 405g hard cast lead projectiles pushed by 10grains of Unique powder. For reference 1lbs is equal to 7000 grains so should get 700 rounds out of a pound of powder which makes it very economical in terms of 45/70. Gives a very light recoil and sub sonic so it isn’t as annoying to people near by (and wives). Even with the sun sonic we get surprising penetration from it. This is into freshly cut poplar logs in -15 conditions.
