Headspace,
325gr isn't a big, heavy bullet in a 45/70 or a .500 for that matter. I shoot 325s from my 44 mag handgun.
Heavy for caliber would be a bullet heavier than the "standard" 405gr, as in 45/70/405.
So a 525gr bullet is big and heavy from a 45/70.
And this is the wierd thing about the .500 S&W, and almost all of the .50s come to think of it, they are mostly 'express' setups, and shoot "light for caliber" bullets, rather than heavy for caliber.
If a .459 550gr bullet is considered heavy for caliber, the 9% larger diameter .500 should be shooting something upwards of 600gr, but I don't know if I've ever seen a bullet that heavy for a .50, while there are 45/70 bullets that exceed 600gr, and the percentage equivilent for the .50 would be 654gr. You have to go to 50 bmg to see those kind of weights.
Please don't think I am dumping on the .500 S&W, I'm not, but I am quibling about the idea of 325gr being a big heavy bullet in the context of the original question.
Regards,
Grizz