yeah I agree that the 4506 was bulky being the second gen, but the 4506-1 has a slim third gen frame and in yr case the 4566 is also a third gen.
here is some SW info:
First two digits - caliber and frame type
39xx - 9mm single stack magazines
59xx - 9mm double stack magazines
69xx - 9mm compact, double stack magazines
40xx - .40 S&W
10xx - 10mm auto
45xx - .45 ACP
Third digit - trigger type and frame size
xx4x - DAO
xx5x - DAO, compact
xx8x - DAO
xx2x - DA w decocker
xx3x - DA w decocker
xx7x - DA w decocker, compact
xx0x - DA
xx1x - DA, compact
xx6x - DA, large frame
Fourth digit - Frame material (all models had stainless steel slides)
###3 - Aluminium alloy frame
###6 - Stainless Steel frame