Problem printing bigger models in the Ender 5
This sounds like a bed leveling issue.  As reported by others, I get much less warping with PETG than with ABS or even PLA, and deal with too much adhesion with PETG rather than too little.  With a Reprap x400, I printed a faceplate for the extruder to hold a electronic drop indicator.  This gives me much higher precision leveling. Of course, I remove the drop indicator after leveling.
If leveling isn't the issue, then you may be printing too fast.  The recommendations I've seen are to print PETG at 50 mm/s or less.  I print a a lower speed than that.