I think it is exact the same size like before after you nulled it, because it is maybe cached by the appl. or something like that. Seems it accepts the writing 1st, but a file handle appears to be still open on that file that revokes the former action somehow. I am no programmer, maybe someone else can explain it in detail.
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Expand FS means increasing the size of the mount?
Yes, if that's an option for you. The appl. might crash, spit errors or something like that if it runs out of space anyway.
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I cannot both rely on removing the file or expanding the file. The only solution I can go for is a safe cleanup of this file, without removing or renaming.
You misunderstood me. When you are allowed to get a short downtime maybe once a week, depending on how fast the log is growing, or once per night, whatever, you stop the application graceful, cp that file to a.log.`date %+w` for example, compress it and just null the original like you tried already "> a.log". When your log files are set, start your appl. again.
Write a little script for this and put it as a cronjob - done.