You can determine the filesize with something like:
> ls -l | tr -s " " | cut -d" " -f5
You can learn the filename for the day before with something like:
>find * -mtime -2 -mtime +1
And you are left to combine the logic of the two:
>tfile=$(find /<path> -type f -name "eon*.tar.Z" -mtime -1)
>yfile=$(find /<path> -type f -name "eon*.tar.Z" -mtime -2 -mtime +1)
>tsize=$(ls -l "$tfile" | tr -s " " | cut -d" " -f5)
>ysize=$(ls -l "$yfile" | tr -s " " | cut -d" " -f5)
[I think I have all of that syntax correct
]
Now, do your math with $tsize for today's file and $ysize for yesterday.
And execute whatever corrective actions are needed.