Easy-rsa[1] (OpenVPN's key/cert generation/signing tool) version 3 will generate a 2048 bit diffie hellman key by default. Previous versions of easy-rsa used 1024 bit as default.

Here is a comment written in the vars configuration file for easy-rsa 2.2.2:

    # Increase this to 2048 if you
    # are paranoid.  This will slow
    # down TLS negotiation performance
    # as well as the one-time DH parms
    # generation process.
    export KEY_SIZE=1024

So if you used easy-rsa version 2.2.2 or previous to generate your diffie hellman key for the server, and didn't increase the default size in the vars file before doing so, your server uses a 1024 bit diffie hellman key.

[1]https://github.com/OpenVPN/easy-rsa