AutoCAD file size

Off the bat, I would recommend adding the title block with Inventor, then saving as dwg, and cut out the AutoCad transfer.

I found Inventor great overall, but it does have quirks with exporting/importing, and some other things. Version 10 improves on some of that. I haven't used the newest version, but upgrading might smooth things out, if you have some extra money you want to throw around. I have to use Pro/E now and hate the interface!

If you want to keep your current process, here are some things to look at. Check the output settings in Inventor and make sure you are 100% consistent from drawing to drawing. There are settings that keep 3D data when exporting 2D drawings that will increase file size. There are various 2D settings that depend on what you do while in 3D mode. Unless you are 100% consistent with your modeling process, some of those might be changing. Make sure everything is on one layer. In AutoCad, make sure you are adding the title block to the same layer as the drawing, or at least stay consistent with layer settings if you want it in a separate layer. Depending on how you are doing the importing and title block, you might be accidentally adding layers that would increase the file size. Check that the drawing sheet size isn't changing somewhere along the line.

It has been a while (years) since I've used 2D anything, but I seem to remember there being a line height sometimes included when data from a 3D program was imported. It would show up as one layer, but multiple heights. That would definitely add to the file size and might be easy to miss. There is some way to force everything onto one plane, or project onto one layer, etc. that fixed it. You also might be getting multiple instances of each line that would multiply the file size. Some obscure setting might cause something like that.

With picture output from Inventor, the exported file size depends on the size of the model window when you tell it to output. I don't see any way that a dwg output would do the same, but checking that dumb things like always having the window maximized are consistent might surprise you. And frustrate you, make you curse Autodesk, your computer, the world, etc for the stupidity of it and the time it cost you :)

Just adding to Butchers comments, once you have the 2D AutoCAD file you can use the 'purge' command to remove any redundant layers, linetypes, dimstyles etc. If you need to upgrade Inventor you can now get an Inventor LT version which is substantially cheaper, I think its main limitation is that you can produce parts but not assemblies.

If you update Inventor to 2008 you can save in the .dwg instead of .idw file type, keeping the information native to AutoCAD. We use it because some of our customers and outsourced vendors have AutoCAD 2000-2008.