How to reduce the file size, if it doesn't reduce by using Purge & Audit?

AutoCAD® DGN Hotfix

This Hotfix cleans up non-essential data from DWG files that have become bloated while copying and pasting from DWGs containing DGN linetypes. It will also prevent future occurrences of the file bloat issue from occurring.

Delete unused geometry

Find Invisible objects and remove them

Delete unused layers

Delete unused Layer filters

Check your Text and Dimension styles

Use overkill command to remove the duplication

Use Purge Command

Purge Regapps

Audit

Recover

Wblock

Use External references for Blocks and some other common shape

These all points will be help you to reduce the file size. 

Then if you need to upload or attached the file you can zip before attaching or uploading. 

zip it with win zip  or winrar .