On one machine in my lab, /etc/rsyslog.d/xenserver.conf defines 17 outchannels, every one of them at 104857600 bytes, and /var/log/messages isn't one of them; it's written by the stock *.info;mail.none;authpriv.none;cron.none line in /etc/rsyslog.conf and rotated by /etc/logrotate.d/syslog, so nightly and with no size trigger.
The docs at https://docs.xcp-ng.org/guides/logs#rsyslog describe the 100 MiB rule but don't list which files it actually covers, which is probably why this is hard to work out without reading the config the way you did.
Whether messages can realistically outrun a nightly rotation I don't know, and I'd be guessing if I said either way; the one thing in your favour is that /var/log is its own filesystem, 3.9 GB on the box I looked at (small lab machine, I haven't searched in the code, so I don't know where that size is decided), so it can't take the rest of dom0 with it.
Whether that omission is deliberate or just inherited (or even something fancier) is really a question for whoever owns that file, so it might be worth a mention to @Team-OS-Platform-Release.
