Longer bullet needs a faster rate of twist to stablize, and especially at longer distances when the velocity drops off. Crosswind can raise holy ned with some bullets when the distances start getting past 3-400 yds as well.
Using heavy smokeless charges or heavily compressed charges of 3f blackpowder in the 45-70 improoves the rate of success at distance with the postel, but there's a price in recoild to be paid for that.
You also get a rate of deminishing returns by going light (short) with the bullets , the original 1.1 inch length (405 +- grsx.) will shoot decent to 800 yds at times but most of the time starts to fall apart at 600.
Think of it like a spinning top. When the top is spinning at it's highest rpms, it stays nice and steady, when it starts to slow down it starts to wander, and when it gets to slow it wobbles around a falls over. Bullets do the same thing.