It would have been nicer if they followed Nissan (or basically any other japanese manufacturer) with their nomanclature which designates series, displacement, valvetrain, fuel injection and number of turbos RB26DETT, VG30DETT, SR20DE, etc.
The block numbers come from changes to bore and stroke. Normally the lower numbers are the smaller motors, just because its easy to start small and work your way up. The block often slightly changes when going up in number, but its generally the same. (the 3s and 5s blocks are almost the same on the outside)