The gelatinous cube boggled my brain as a kid. As my fellow players and I were eating lime flavored jello cubes our moms had made in the summer, it wasn't hard to imagine what one of these would look like full sized in a dungeon. Why or how they adapted to fit perfectly into a 10' wide dungeon passage (mirroring our graph paper's 1 sq=10') never entered our minds, nor apparently their creator's (Gygax). I reckon we unconsciously thought that scarfing these down was the monsters' just desserts.
This fantastic image of a Gelatinous Cube was created by Shockbolt at Deviant Art.