DB mail sending attachment of size 64 KB only even after increasing maximum file size limit
You need to apply Cumulative update package 4 for SQL Server 2014. There was a bug which was fixed in CU4.

Symptoms


  Assume that you use Database Mail and sp_send_dbmail stored procedure
  with the @query and @attach_query_result_as_file option in Microsoft
  SQL Server 2014. The stored procedure run a query and set the
  result-set as an attachment of the email. When the query has a large
  result-set, the attachment is truncated to 64 kilobyte (KB) maximum.


Resolution


  The issue was first fixed in the following cumulative update of SQL
  Server.


Details in below link:

FIX: "sp_send_dbmail" stored procedure truncates email attachments to 64 KB in SQL Server 2014