SQL Server Varbinary(max): select a subset of bytes from the varbinary field

You use SUBSTRING. This reads a snippet from your varbinary data on the server, and only returns the snippet to the client.

[SUBSTRING] Returns part of a character, binary, text, or image expression in SQL Server.. [it] Returns binary data if expression is one of the supported binary data types. The returned string is the same type as the specified expression with the exceptions shown in the table.