in sharepoint 2013, 2016 , there is a limitatoin on the size of the uploading files , default size for uploading documents through CSOM is 2 MB, but you can overcome this limitation by using the following PS script to set a higher limit
$ws = [Microsoft.SharePoint.Administration.SPWebService]::ContentService
$ws.ClientRequestServiceSettings.MaxReceivedMessageSize = 10485769
$ws.ClientRequestServiceSettings.MaxParseMessageSize = 10485769
$ws.Update()
Note : stop sharepoint timer service before executing above script
SharePoint 设置客户端上传文件大小
原文地址：https://www.cnblogs.com/jianyus/p/8390673.html