Patrick Smacchia, of NDepend fame, has done a detailed, low level look at exactly what changed in .NET Framework 3.5 SP1. 

Some data he lists:

# Assemblies    112
# Namespaces    919 to 935      (+16   +1.7%)
# Types    39 988 to 40 513      (+525   +1.3%)
# Methods    387 421 to 386 790      (-631   -0.2%)
# Fields    241 567 to 246 795      (+5 228   +2.2%)
# IL instructions    8 598 933 to 8 620 940      (+22 007   +0.3%)

Overall, for the amount of new functionality we added in SP1, I think this seems like fairly modest growth of the framework.

Patrick also includes some cool eye charts to help visualize the changes...  I am not sure you'll see this poster as a marketing give away at the next conference, but it is pretty neat to look at.. 

Enjoy!