From Brickipedia, the LEGO Wiki
Jumbo Bricks was a small theme exclusively manufactured by Samsonite from 1964 to 1972. The sets contained bricks larger than the normal system bricks, much the same as today's DUPLO bricks. However, unlike DUPLO, Jumbo Bricks are not to a relative scale to standard LEGO bricks. They were marketed towards children from nursery school through to first grade. There were six sets produced for this theme.