I have imported dwg models in sketchup, cleaned them up (removed excess lines, put in layers, made groups and components) so I could work with and made little adjustments to the model in sketchup and exported it again to dwg. The dwg export however is huge, like several hundreds MB.
It was a pretty heavy model to begin with, with loads of meshes. But still : the original dwg file was like 20MB, and after adjusting it in sketchup and exporting it again, it is about 10 times as big.
Are there things to avoid in sketchup when you want to keep the dwg size small?
It is helpful to keep the elements as simple as is possible. I try to avoid using groups and much prefer to make repetitive objects into components. Also, refrain from nesting too many entities within other components when possible. I prefer to explode certain elements and remake them into new components. I also find it helpful to edit my models by copying and pasting items into a separate model using paste in place (this is particularly useful when the original model is fairly large), then running the CleanUp3 plugin by ThomThom after finishing the edit, then re-pasting in place back to the original model. Many of these steps are aided by using shortcut keys.
I periodically confirm the overall size of models as part of the workflow process and if I notice the file is increasing substantially, I run CleanUp again. If the model size is still too large to suit me, I will break tthe original model down into more manageable parts (as several single but interrelated models). Each single model can now be treated independently of the others and can be separately exported into a dwg file. The same dwg file can receive a number of imported models and will be equivalent to the overall dwg file that is derived in the typical fashion.
Have you checked if running the Purge Unused command (Window menu>Model Info>Statistics) helps? Importing a CAD file often results in a zillion unnecessary component definitions that will, again, become a zillion block definitions when exported to DWG.