When the plugin was created I got the info that ESX thin disk support is only available as a filesystem feature, not the disk image format itself. Maybe it has changed since then.
It seems that ESX support also personal disks, so you can use it in your case. One of our community members crafted this tutorial. In your case use:
boxgrinder-build netbooter.appl -p vmware --platform-config type:personal,thin_disk:true
And please, let us know if it booted correctly, thanks!
Hello Marek and thank you for the tip, it creates a nice small portable file instead of the initial raw 16Gb file.
And it boots nicely in an ESXi environement. So thanks again for taking your time!