Thumbnails for NEF are huge, 10-15 MB

When I open a directory with photos of Nikon NEF format, then gnome-raw-thumbnailers takes over most of my CPU and generates large files: A 20MB Nikon NEF image results into a 10-18 MB thumbnail generated in PNG format in .cache/thumbnails.

Thumbnails for NEF did not show by default in 18.10. I followed Nautilus isn't displaying thumbnails for my NEF files (photo raw)

As a resuilt the thumbnail directory is almost as big as my Pictures directory. How can I reduce the size of my thumbnails?

ProblemType: Bug
DistroRelease: Ubuntu 18.10
Package: gnome-raw-thumbnailer 2.0.1-0ubuntu9
ProcVersionSignature: Ubuntu 4.18.0-11.12-generic 4.18.12
Uname: Linux 4.18.0-11-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.10-0ubuntu13.1
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Thu Nov 29 09:04:35 2018
InstallationDate: Installed on 2018-08-12 (108 days ago)
InstallationMedia: Ubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
SourcePackage: gnome-raw-thumbnailer
UpgradeStatus: Upgraded to cosmic on 2018-10-23 (36 days ago)