Yes, I reckon it is all of that.

According to my Hodgdon reloading ,manual,the 45-70 has .608 rim & .505 base, 45 Colt has .512 Rim & .480 base. With a bit of leeway from tolerances, ie min dimension Colt cases & sloppy 45-70 chamber the Colt would nearly disappear into the 45-70 chamber, assuming that the Colt case was held by its rim, you would at the least get massive case bulging if not slitting & hot gases spraying around all sorts of places they weren't meant to.

Steve