How to completely save large chat history of WhatsApp in Android phone
There is a limit which is imposed because of the maximum sizes of emails

see the WhatsApp FAQ section of their website


  If you choose to attach media, the most recent media sent will be
  added as attachments. When sending with media, you can send up to
  10,000 latest messages. Without media, you can send 40,000 messages.
  These constraints are due to maximum email sizes.


whatsApp FAQ Android