Problem reported in the Debian BTS by Kevin Ryde 8 years ago, still applies to latest xmessage:

I noticed that xmessage can get its initial window size wrong when
displaying a message containing tab characters.  For example, create a
file containing
        <tab><tab><tab>hello world
and run
        xmessage -file foo

and it shows scroll bars rather than making its window big enough.

Patch below to convert tabs to spaces in the message, both
for file and command line messages. ab stops are 8 characters (hard
coded), newlines are understood.