Gmail does not support sending files bigger than 20MB. this can happen in other email clients as well
Solution 1: Choose HTML as the export format
HTML is a much smaller file, incase it is still too big you can split it to multiple files (see solution #3 for instructions)
Solution 2: Share via Google Drive
Instead of gmail (or any other app), choose "Save to Drive"
Scroll the notification tray from the top - you will the the upload progress of the file. when the file is fully uploaded to Drive, click on the notification
Press the 3-dotted icon next to the conversation file
Turn on "Link sharing" and press "Copy link"
Open the your mail client app, write a new mail and paste the link in the mail body.
Solution 3: Split the file into multiple smaller files
In the sharing dialog click on settings icon next the the selected format
- Turn on "Split Files"
- Choose how many messages in every file
- Save (on the top-right corner)
Solution 4: Connect your desktop computer to your phone via USB
You can transfer the files to your desktop via USB.
Just connect your phone and browse to "WhatsApp Exporter" directory in your phone's internal memory