This image shows a Samsonite (left) and European (right) Jumbo Brick. A modern 2×4 brick is included for comparison.
The Jumbo Bricks were first made by Samsonite in 1964 (pat pend). A European version followed in 1966 (without pat pend mark). These bricks were abandoned when Duplo was launced in 1969.
When you scale up a regular brick to Jumbo size, the walls become very thick, heavy and expensive. Samsonite solved this problem by making the studs larger. LEGO Denmark apparently did not like that design and made some adjustments. The studs are left in approximately the same proportion to the brick, and ridges are made in the inner walls to make sure the bricks connect to the walls.
Many years later (after the 1973 oil crisis) LEGO would apply the same design to the regular system bricks. The walls were made thinner and inner ridges were added.
[Original post and comments on Flickr.]