changing cipher for imap clients

When my client connects, I see this in my log:

dovecot: imap-login: TLSv1 with cipher ECDHE-RSA-AES128-SHA (128/128 bits)

Whereas, when client connects to my postfix server, I see:

Anonymous TLS connection established from * TLSv1 with cipher ECDHE-RSA-AES256-SHA (256/256 bits)

how can I tell dovecot to use AES256, instead of AES128 ?

is this set by ssl_cipher_list ? Here are my current values (defaults)

# doveconf ssl_cipher_list ssl_cipher_list = ALL:!kRSA:!SRP:!kDHd:!DSS:!aNULL:!eNULL:!EXPORT:!DES:!3DES:!MD5:!PSK:!RC4:!ADH:!LOW@STRENGTH

# dovecot --version 2.3.4.1

thanks,