I'll start by saying that this isn't a problem, just curiosity.
I have some .NET dlls in my program, and I'm seeing a 200-500 % size increase for the pdb size.
From that, I'm trying to do a back of the envelope calculation for how much code is in a few of our third party libraries, because I have a suspicion that it's "way too much" (in terms of how much code I estimate it would take me to write the same thing). Reflector crashes on my machine when dealing with these dlls, but because this is curiosity it's not really a priority to figure out why.
(I know that conventional wisdom says "lines of code" isn't necessarily the greatest metric, but it can be useful: i.e. if a "Hello World" program takes 500 lines excluding framework, something is definitely wrong)
There is no direct correlation between DLL and PDB size, but there may be a correlation between number of IL instructions in DLL and PDB size. It's fair to expect that one 1 KB worth of ternary operators would produce larger PDB than 1 KB worth of code that writes formatted string to console.