Gallery Block, Three columns style: Uses full images instead of thumbnails

While testing the automatic conversion of the old ThreeColumns block to the new Gallery block, an issue became apparent: 

While the old ThreeColumns block used a smaller version of the image, the new Gallery block (while on the threeColumn style) uses a full version. This results in much bigger images than required (in a case: 1MB instead of the old 100kb). 

Example (old block): 

https://release.k8s.p4.greenpeace.org/brasil/explore/inspire-ativismo/ https://k8s.p4.greenpeace.org/brasil/explore/inspire-ativismo/

Task: in the ThreeColumn style of the new block, use smaller version of the image (use srcset etc as we were doing in the threeColumn block).