See the differences in size between common sensor formats. Put simply, the bigger the sensor, the shallower the depth of field for any given ‘composition’ in the frame. And the wider the field of view of any given lens. So a 15mm lens on full frame shows you more of the world than on cropped sensor. Check out the field of view comparator for a visual demonstration.

Common Focal lengths as rendered to a Full Frame 35mm sensor like the Canon 5Diii