Need much more information to get an idea of what the problem might be.
What brand and release of Linux?
What version of HP-UX?
Your mount command shows no -F file system type or -o mount options. Did you use any?
What where the options you used to mount the file system on HP-UX?
What type of Linux file system did you create to receive the 18GB file?
Do you have any file size limited set in Linux:
ulimit -a to view "file size" settings
/etc/security/imits.conf for fsize limits.
What are your Linux export settings?
What is the hp-UX "bdf /dir1" result and Linux "df /dir2" result?
I've copied much larger files with this cross-mounted file systems RHEN 6.5 ext4 to HP-UX 11.31 with these settings;
HP-UX /etc/fstab: (Linux IP) ###.###.###.### :/transfer /transfer nfs rw,bg,hard,intr,rsize=32768,timeo=600,noac,forcedirectio 0 0
RHEN /etc/exports: /transfer (HP-UX IP) ###.###.###.###(rw,all_squash,sync)