Buck,
The core is swaged, but then electroplated with copper or a copper alloy. For example, the Speer Gold Dot line is made this way and one of their technicians told me they engrave in rifling more easily than jacketed bullets and can often take more powder because of the resulting lower start pressure. The Ranier and Berry's bullets are also made this way, but I believe they use pure copper. Also, Berry's makes them available with extra heavy plating. These will all behave a little differently.
Kmw,
One factor in changing bullets is length. This affects how deeply the base sits in the case. Below is a photo of a pressure calculation. I believe Western has a pressure change graph for an example of a 9 mm bullet seated to different depths, to. So when you take the different bullet length and COL combinations into consideration, that's a good part of why you run into a lot of different load data for same-weight and construction bullets.
In the short powder column pistol bullets, the fact primers can unseat bullets and move them out before powder finishes burning will reduce pressure. Look at Hodgdon's data for 148 grain wadcutters in .38 Special and .357 Magnum over HP38/231. The Special actually has a larger powder charge. I verified with Hodgdon that this was a measured result they have on file and not a documentation error. The bullet getting moving ahead of burn completion is the only explanation I have come up with thus far. Bottom line, that primer unseating has probably prevented damage from hot loads where the trip up the loading ramp has jammed a bullet deeper into the case.
Poconolg,
Do the following calculation for your old bullet and the new one. If the number comes up shorter for the new bullet, this will explain why you need more powder.
Seating Depth = Case Length + Bullet Length - Cartridge Overall Length (COL)
Or, more specific to your cartridge:
.45 Auto Seating Depth = 0.898 inches + Bullet length - COL